A review
Historical evolution of preformistic versus neoformistic (epigenetic)
thinking in embryology

Marc Callebaut

UA, Laboratory of Human Anatomy & Embryology, Groenenborgerlaan 171, B-2020 Antwerpen Belgium To the memory of Professor Doctor Lucien Vakaet sr

Corresponding author : Prof. Dr Marc Callebaut, Laboratory of Human Anatomy and Embryology UA, 171 Groenenborgerlaan, B2020 Antwerpen. Phone: ++3232653390; fax: ++3232653398; e-mail. marc.callebaut@ua.ac.be.

ABSTRACT. In the classical embryology there exist two main concepts to explain the rising of a living being. At one hand there exists the theory of preformation i.e. all the parts of the future embryo would already exist preformed during the preembryonic or early embryonic period. On the other hand there is the epigenetic view in which it is propounded that all parts arise by neoformation from interacting (induction) previously existing, apparently simpler structures. We found evidence that initially a kind of preformation (or pre-existence), disposed in concentric circular layers, exists in the full grown oocyte and early germ. After normal development these circular layers will settle successively, from the centrum to the periphery, into the central nervous system, the notochord, somites, lateral plates (coelom) definitive endoderm. Under influence of mechanical and gravitational forces an early epigenetic development starts by unequal oblique uptake (unidirectional chaos) and segregation of ooplasmic determinants in the germ. So an epigenetic cascade of early general body plan formation occurs in the vertebrate embryo. Recently, we demonstrated by hemi-sectioning of avian unincubated blastoderms that both a kind of preformational (mosaicism) as epigenetic (regulation) behaviour depends on the spatial, ooplasmic distribution of Rauber’s sickle material, homologous to the also sickle-shaped Wnt expressing gene region in ascidians. This clearly brings preformation and epigenesis much closer together since there is a common influencing ooplasmic factor. An ooplasmic continuity bridging the premature oocyte ooplasm to the embryonic primordial germ cells of the following generation is demonstrated. Both nuclear and ooplasmic continuity in the oocytes, present in the ovary of birds and mammals, is shown by radioactive premeiotic DNA labelling.

KEY WORDS : preformation, mosaicism, epigenesis, predispositions, ooplasmic determinants.

INTRODUCTION

Among all biological topics, embryology (developmental biology) presents most uniformity in the animal and even in the vegetal kingdom. So, not with standing the pronounced structural differences between species, it forms a common link among all living organisms. All embryological processes are based on cell biological, genetical and biochemical principles since every organism is formed by isolated or associated cells. In embryology one studies the very complex processes by which these cells differentiate and associate from one single initial more or less voluminous cell: the egg. The egg is the link between two generations and already by its volume it gives the impression to contain more then an ordinary cell. Our mind needs, already from its earliest manifestation, to know how a young animal, a child or a plant makes its appearance. It is still one of the first questions that children ask for when they begin to think and realize their own existence. Moreover their own origin, as is the case for most mammals is most difficult to observe since the mammalian embryo is completely hidden from direct observation. By contrast, avian embryos are more accessible to direct observation. For instance chickens lay eggs, incubate them by their own body heat and after approximately 20-21 days, as touched by a magician, hatching of a young bird takes place. It is easy to open the eggs at the successive stages of development, sometimes even without killing the embryo. This has been and is still one of the bases of modern embryology.

EARLY HISTORY OF IDEAS
IN RELATION WITH THE ORIGIN
AND NATURE OF THE GERM

Ancient Arabs and old Germans thought that only mothers have the possibilities to give origin to the development of a child, whilst people in Greece considered that the father was only responsible (SHORT, 1977). Greek natural philosophers generally interpreted female seminal fluids as menstrual blood. The female usually was considered inferior to the male: due to a lack of internal heat, this menstrual blood did not form true seed. This did happen in males, leading to male seed or sperm, which also was thought to form out of blood (generally conceived as a life giving principle). Hippocrates (460-380 B.C.) “the father of medicine” was the first to control theories with real experiments. He made from embryology a separate science and made comparative embryological studies between the chicken and the human embryo. He accepted that both the female as the male contributed equally to the formation of the germ. ARISTOTELES in the 4th century before J.C., also called “father of the natural history”, was the first to describe the “punctum saliens”, the early beating heart of the avian embryo and followed its evolution, day by day during further incubation. While he observed the evolution of the chicken embryo and it’s appearing organs, he concluded that the organs appear successively and not at the same moment. Therefore he proposed the so-called epigenetic development in contrast to the preformation hypothesis. Aristoteles did not consider uterine blood but the male seed as the essential factor for germ formation. Aristoteles saw it as an unmaterial principle of movement “triggering” the menstrual blood to further selforganize. The female “seed” is to be considered as the material cause of any embryological process. Ancient people must have observed the existence of egg sacs with large eggs in the ovaries from birds. The yellow aspect of the large follicles in the ovary and a similar aspect in the laid egg were the first indication that eggs are formed in the ovary. So the link between ovarian oocytes and the avian germ in the laid egg (oviparity) was established. But the principle of viviparity (development of an embryo in the female organism) could originally not been explained. ARISTOTELES in the 4th century before J.C. has made embryological observations on much different kinds of animals. He followed not only methodically the development of the chicken embryo but has also observed the embryos of dolphins and larval stages of insects (CAULLERY, 1957). The role of the male and female respectively in the generation process could be explained according to Aristoteles by the example of the chicken. Indeed, the latter taken apart from the rooster still continues to lay eggs in which however no embryos develop. Therefore he concluded that generally speaking the female furnished the constituting material of the embryo whilst this material was vitalized by the male. The place where this occurred in the chicken egg would be the cicatricular region where the germ disc, forming the base of the future embryo develops. Departing from Aristoteles’s natural philosophy, VAN SPEYBROECK et al. (2002) have historically shown that epigenesis gained alternating attention from the 17th century onwards. It was considered to be the opposite of the preformationist tradition. Where preformationism stated that the germ cells of each organism contain preformed miniature adults that unfold during development, epigenesis held that the embryo forms by successive gradual exchanges in an amorphous zygote. CLAUDIUS GALENUS (131-201) one of the founders of anatomy and physiology was born in Pergamon. He made dissections of animals, mainly apes, because it was not allowed to study human corpses. His influence has been very important until in the 16th century. Galenus described the female ovaries as testes and propounded that the male sperm was secreted by the testes. So the notion of gonads (genital glands) male or female was established. Galenus holds a very naturalistic view on the embryological processes in terms of growth, nourishment and genesis (change and shaping). After Galenus during 13 centuries, science and particularly embryology were completely influenced by religious ideas and no new discoveries were made.

PERIOD
OF THE EARLY ANATOMISTS

Fabricius Ab Aquapendente (teacher of William Harvey) had a chair of anatomy at Padua and made an enormous contribution to embryology (ADELMANN, 1942). He

Fig. 1. – Preformation concept during the 17th century: drawings of the early developmental stages of the chicken embryo in which, according to Fabricius Ab Aquapendente, already the adult form could be seen.

was probably the first investigator to give an exact account of the role of the ovary in the formation of the hen’s egg, for he observed that besides that the yolk was formed in the ovary that the egg white, the shell membrane and the shell were all formed during the transit of the egg down the oviduct. This key observation was the first indication that the egg might be produced directly by the female rather than as the result of the union of male seed “and female soil” as supposed by Aristoteles. He also thinks that the avian egg is fecundated by the so-called “aura seminalis” (an emanation from the semen of the rooster). FABRICIUS was also a preformationist since he pretended that a “little birdie” was visible in the egg during the earliest developmental stage (1637) (Fig. 1). However the initial developmental stages of most of the embryos have a size which makes them invisible to the naked eye. The first microscopes appear in the 17th century, but only in the 19th century a higher degree of perfection permitted their real use for embryological observations. Embryos in advanced developmental stages are more or less visible to the naked eye and already have more or less the form and structure of the adult animal. This suggested the attractiveness of the theory of preformation i.e. that development was only the result of the progressive enlargement of the germ, having from the beginning its final constitution and complexity. The preformation theory seemed to be first propounded by the arab scientist, AVERROES (also called Ibn-Roschd) who was a teacher at Cordoba (1115-1198). Contrary to this theory, the theory of epigenesis was formulated: the complex structure of the definitive adult forms and even the embryo is only realized progressively by transformation of more simple structures. In the course of the history of embryology, as a research discipline, it appeared that a kind of progressively build up preformation (pre-existence) exists in the egg (already before fertilization), which is indispensable for early embryonic development. Initially, epigenesists defended the view that the embryo progressively forms out from a homogeneous matter.

Aristoteles, but also HARVEY (1653) claimed that it is most certain that in the egg there is no prepared material at all. It takes until the studies of Albrecht Von Haller (1708-1777) first adhering to epigenesis, later becoming a strong ovist-preformationist, to conclude that the egg does contain differences in viscosity.

EARLY EXPERIMENTATORS
AND USE OF MICROSCOPES

WILLIAM HARVEY (1578-1657), medical doctor from the king (Charles I) of England, discovered the blood circulation (1628) and made dissections from female deer’s after coitus and states that the young germs of the mammals are present in the form of eggs. In 1653 appears his book: “Exercitationes de generatione animalium”.

He did point out a place where fecundation would occur, but at the same time, his research on deer did not allow him to conclude that the male semen made any material contact with the egg (or left any visible trace) on the female egg. According to him the little white cicatricula on the top of avian eggs is the place where fecundation occurs. After incubation this region extends and forms a semi-liquid mass (colliquamentum) in which soon the first traces of the embryo and particularly the heart appear. From him comes the aphorism “ex ovo omnia”: every living being develops from an egg (1651). REGNIER DE GRAAF (1672) who was an excellent experimentator (LINDEBOOM, 1973) observed that after killing, female rabbits at different moments after coitus present a progressive evolution in the vesicles bulging at the surface of the ovary, followed by scars after ovulation. These “eggs” (sic) were found afterwards in the uterus. He thinks erroneously that the vesicles bulging at the surface of the ovary before the coitus were really the eggs themselves (SAWN, 1997). He observed that once they were found in the rabbit uterus they are first small but later larger, adhere to and become fixed into the wall of the uterus (implantation). Now we know that the ovarian vesicles described by Regnier De Graaf are not the eggs but are follicles surrounding the real, much smaller eggs. These follicles are now still called Graafian follicles in modern literature. At nearly the same moment, thanks to the use of the first microscopes, spermatozoa were seen in semen (perhaps by HAMM, or HARTSOEKER, 1694; fig. 2) but they were surely seen, drawn and described by the Dutch researcher ANTONIE VAN LEEUWENHOEK (1678; 1683). The main discovery of Van Leeuwenhoek was his finding in 1677 of “little animals” (animalcules) in semen (LA BERGE, 1999). He found these animalcules in semen from men but also from roosters and in the semen of different mammals. So Van Leeuwenhoek became an “animalculist” or “spermist” who thought that later the embryo was formed from one of these little animals (Fig. 2) or spermatozoa (Von Baer used this word). According to Van Leeuwenhoek, De Graaf’s egg, if it did anything, provided no more than nourishment to the embryo. During the next century little further research was done on the topic, although there were much theoretical discussions between the animalculists and the ovists (who considered the egg as the location in which the preformed embryo resides). De Graaf’s ideas on the egg as being the precursor of the embryo fell in some disfavour because the evidence for the animalculists was easy to obtain by simple observation of semen under the microscope, which seemed to demonstrate the animalculist preformation theory (LINDEBOOM, 1973). The ovistic hypothesis of De Graaf was on the contrary more difficult to control. It was in fact only much later, 100 years after De Graaf’s death, that CRUIKSHANK repeated and confirmed his studies on embryonic development in rabbits. Marcello Malpighi (1628-1694) described the capillary circulation in the lungs of frogs and also studied the development of the chicken embryo. His study “De formatione pulli in ovo” appeared in 1672. This biologist used systematically a microscope (fabricated by JANSSEN in 1590). He described the cicatricula as the most important part of the avian egg for the development of the future embryo after incubation. With Malpighi the microscope became an indispensable tool for biological research (LAMS, 1935; LÜTHY, 1996) and he adhered to ovism. In 1740, at the age of 20, CHARLES BONNET (Switzerland) describes for the first time the phenomena of natural parthenogenetic development (from a virgin egg) in a plant-louse. These observations of BONNET on virginal reproduction were confirmed by RÉAUMUR and BEGUELIN, TRIMBLEY and ALBERT VON HALLER. This discovery of parthenogenesis was strongly in favour of the ovist theory of preformation. However, the eventual “emboîtement” of the successive generations could hardly been explained. We must also mention the theory of organic molecules developed in 1745 by MAUPERTIUS (DOLLANDER & FENART, 1973). Seminal liquids both from the mother as from the father, coming from all parts of the body, should mix after mating in the uterus. So, Maupertius tries to explain that some characteristics of the father or mother are recognized in the children. By contrast both ovists and spermatists (or animalculist) believed in the theory of preformation i.e. that the egg or the spermatozoon contains a miniature individual with all the parts of the adult. FABRICIUS AB AQUAPENDENTE (1637) and MALPIGHI (1669) were convinced preformists and according to them the avian germ disc contains a miniature organism already in its definitive form (Fig. 1) (ADELMANN, 1942; 1966). By growing, its different parts unfold progressively into an adult form. Albertus Von Haller (1753) had a more evoluated idea (with epigenetic background) about preform-ism. He wrote: “nulla est epigenesis, nulla in corpore animali pars ante aliam facta est et omnes simul creatae existunt”. He found no recognizable organs in the chicken germ but considered that only their Anlagen were present and that these will progressively develop into their adult form. Von Haller was an authority in embryology and studied the chicken embryo at early stages after incubation. He wrote a book in French and described the development of the heart, the eye and respiratory phenomena (see the title page of his book: Fig. 3). He uses the chicken embryo to explain the formation of some anatomical structures in the human.

Fig. 2. – Drawing of the microscopic aspect of a human spermatozoon of animalculist Hartsoeker, in which he seems to see a miniature baby as was also accepted by Van Leeuwenhoek.

EVIDENCE FOR EPIGENETIC
DEVELOPMENT

The old preformistic view dominated in embryology up to the middle of the 18th century. During that period a new view on embryonic development was introduced by C. F. Wolff (1734-1794). He was born in Germany where he started his research in anatomy and embryology. This resulted in the publication of a dissertation entitled “Theoria Generationis”, published in Germany in 1759. In this study, C. F. Wolff developed and defended a so-called epigenetic view based on careful microscopical observations of early embryonic development. According to Wolff the chicken embryo is not formed by the accumulation of juxtaposed parts but the organs arise progressively the one after the other from more simple structures. Thus the gut is composed first from a flat membrane which later forms folds and finally has the general cylindrical aspect of a digestive tract. In the same way he describes the development of the neural groove followed by the medullary cord. So in the early germ nothing exists “in facto” but all “in potentio”. Thus from already pre-existing structures new and more complicated structures and organs were formed.

Wolff did not so much stress the existence of pre-existing structures. He did argue in favour of “an active nature”. This is however contrary to the preformistic concept of nature as a dead mass unto which blind mechanical forces work. It is in this regard that Wolff talks about “inorganic matter”, as the heterogeneous, but still unorganized, matter out of which an embryo self-organizes. At that time however this theory of epigenesis was not accepted in German scientific circles. So Wolff could not find a place in the German universities and immigrated to Russia. At the end of 1766, the St. Petersburg Academy of Sciences invited him to work as academician in the anatomical department. Finally he settled in St. Petersburg where he worked in the field of embryology, teratology and anatomy. The discovery of organic transformation and neoformation by C. F. Wolff was misunderstood by his contempories. According to Blumenbach’s unordered matter does not have the power to order itself, thus life cannot spring from non-live. The organization one sees in life, is due to a physiological impecunious principle of internal correspondence (Bildungstrieb), ungraspable to the human ratio. This principle is not equal to the mechanical formative power or Bildungskraft that inorganic matter also possesses. Blumenbach’s nisus formativus or “Bildungstrieb” is not entirely the same as Wolff’s vis essentialis. Whereas Wolff’s “wesentliche Kraft” is single in nature, producing but one effect, varying only through the influence of the surrounding context, Blumenbach’s nisus formativus was a multiple active force which could produce many different things by itself, making it by itself sufficient to generate a new organism. This makes Blumenbach much more into a vitalist than Wolff.

For Wolff the vis essentialis produces the different parts of the organic body no longer merely through itself and according to its nature, but rather with the help of countless other concurring causes, “and what it does through itself alone, becomes a completely simple effect, as attraction or repulsion, and is worlds apart from the building of organic bodies” (WOLFF, 1789 in ROE, 1981: 117). This point is very important, since it was and still is often thought that Wolff deduced the total formation of matter from the vis essentialis. Even his opponent, Von Haller, did not grasp this point fully: ”why does this vis essentialis, which is one only, forms always and in the same places the parts of an animal which are so different, and always upon the same model, if inorganic matter is susceptible of changes and is capable of taking all sorts of forms? Why should the material coming from a hen always give rise to a chicken and that from a peacock give rise to a peacock? To these questions no answer is given” (VON HALLER in NEEDHAM, 1959: 202). Wolff asserted several times that people paid too much attention to his vis essentialis and that his theory of attraction and solidification would have been the same without it. A follower of Wolff’s epigenetic theory was TREDERN DE LÉZÉREC, a usually forgotten pioneer of chick embryology. He submitted his thesis in Jena (1808). The importance of the male semen, next to the female egg for reproduction was shown during the same period by Lazarro Spallanzani (1729-1799). Using amphibian eggs, SPALLANZANI applicated the artificial insemination method used by JACOBI (published in the Hannover magazine in 1763) for fecundation of trouts or salmon. This method of Jacobi is still used at present to populate waters. Spallanzani did not believed in the “aura seminalis” theory of Harvey, as he was convinced of the necessity of material contact between egg and semen (more specifically spermal fluid). He demonstrated that the fecundation capacities of the semen of an amphibian (Rana temporaria) disappears after filtration or heating. That fertilization is the result of the effect of spermatozoa on an egg was concluded by PREVOST and DUMAS in 1824. They repeated the experiments of Spallanzani and demonstrated unequivocally that the spermatozoa are the real fecunding elements in the semen. The idea that the spermatozoa play a major role in fertilization by penetration, as was propounded by Prevost and Dumas, was not accepted by all biologists in that time. TH. W. BISCHOFF (1807-1882), in 1842 and J.P. MULLER (1801-1858) in 1844 doubted about this. Bischoff claims in “Entwicklung des Hunde-Eies (1845): “ich habe nie im Inneren eines Eies einen Spermatozoide auffinden können. Die Wirkung des Saamens auf das Ei halte ich dann zunachst fur eine chemische”. By contrast, L’ALLEMAND believed strongly that fecundation consist of the union of two living parts: “un fluide ne peut évidemment transmettre la forme de la vie qu’il ne possède pas”. The fusion of spermatozoa with eggs has been really observed only for the first time clearly in Ficus by THURET (1854). The development of the embryology has also been influenced by evolutionistic concepts during the 19th century by LAMARCK (1802) and DARWIN (1859). In succession of F. MÜLLER (1844), HAECKEL formulated the so called biogenetic fundamental law, according to which ontogenesis summarizes phylogenesis. This means that the development of an individual being is a partial recapitulation of the evolution of his ancestors. The checking of this hypothesis has been at the origin of an important boom in descriptive embryology during the second period of the 19th century. Although Wolff’s epigenetic ideas in embryology were not accepted in Germany, his influence was great on the founders of the Russian embryological school, C. H. Pander and K.E. Von Baer (SANDER, 1996; MIKHAILOV, 1997). Pander, a Russian zoologist born in Riga (12/7/1794) made a thorough description of the developing chick embryo with the three layers forming the body. His schemes of chick embryonic, development at different stages, surprisingly resemble modern classic descriptions. Pander demonstrates for the first time in history that a bird embryo develops from three germ layers (Fig. 4). PANDER (1817) distinguishes clearly in the cicatricula (germ disc region) of the unincubated avian egg the superficial “blastoderma” (now called blastoderm) and the central underlying “nucleus cicatriculae” (now called nucleus of Pander). Pander observed that the tinny membranes which will form the germ also contain a great generative power (nisus formativus) already described by BLUMENBACH (1789). Karl Ernst Von Baer (1792-1876) was born in Piep (Estonia) and, after graduating from the university, he started his career as zoologist and embryologist. He worked as professor of Anatomy at the University of Köningsberg. His microscope was already much better then from his predecessors. Later he worked in the St. Petersburg academy of Sciences, where he worked for nearly 30 years and published more than 400 manuscripts. His scientific work was as he described himself influenced by two persons: “So dürfen die vorliegenden Untersuchungen sich rühmen, eine Folge jener für die Naturwissenschaft ewig denkwürdigen Verbinding zu sein, in welcher ein in physiologischen Forschungen ergrauter veteran (Döllinger), ein von Eifer für die Wissenschaft glühender Jüngling (Pander) sich verbanden um durch vereinte Kräfte eine feste Grundlage für die Entwicklungsgeschichte des thierischen Organismus zu gewinnen” (cited by VAKAET, 1965; page 137).

Fig. 4. – Early composition and disposition of the avian blastoderm and cicatricular region on the avian egg yolk ball, represented by PANDER in 1817: the so-called nucleus of Pander is seen as a triangular mass below the unincubated blastoderm (upper figure) and after incubation below the three germ layers (lower figure), clearly indicating epigenetic phenomena (neoformation).

In 1821, for the first time, VON BAER detected mature unfertilized eggs (oocytes) in the ovary of the female dog. In his paper “De ovi mammaliae et hominis genesis” (1827) he notes that they were identical to the ova found in the oviducts. Later, Von Baer started to analyze ovaries in rabbits, pigs and humans and described the human egg and the structure of the Graafian follicle. Von Baer developed and enlarged Pander’s ideas about germ layers and studied their fates during early embryonic development. He was able to demonstrate that the so-called “cutaneous” layer (external layer) transformed into superficial epidermis and central nervous system, that the so-called “muscular” layer (now merely somites) formed muscles, skeleton and connective tissues; that the internal “covering” layer of the digestive tract developed from the so-called “mucous” layer. From these three germ layers all body structures are formed according to an orderly process. By the study of chick embryos he detected a new embryonic organ for the first time i.e. the backbone “cord” (chorda dorsalis or notocord). Based on detailed comparative studies of embryonic development in different vertebrate embryos he formulated some conclusions: during embryonic development more specified characteristics appear later than the more general features and the general features of a large family of animals appear earlier than the features of a species. Von Baer recognizes a certain similarity only between the early embryos of different animal groups.

Fig. 5. – Schematic drawing representing the onion peel-like localization of the four ooplasms (α, β, γ, δ) at the end of oogenesis (modified after CALLEBAUT, 1975): the ooplasms will play a fundamental role, during the early development of the blastoderm; from peripherally and superficially to more centrally and deeply we see: the α ooplasm which after a centripetal movement functions during the cleavage stage (see Fig. 6); the β ooplasm is mainly incorporated in the primitive streak and in the embryo proper; a part of the γ ooplasm, surrounding the nucleus of Pander (NP), is spatially obliquely taken up caudolaterally in the blastoderm and forms Rauber’s sickle (see also Fig. 8); part of the superficial part of the δ ooplasm (from the nucleus of Pander) is also taken up in a spatially oblique manner by the more central part of the blastoderm and forms the sickle shaped endophyll and primordial germ cells (see also Fig. 8); CHR: spherical postlampbrush chromosomes seen in the centre of the flattened germinal vesicle; SCO: perinuclear subcortical cytoplasmic organelles or ticos; WG: wedge granules layer; PG: polar granules; NL: latebra neck.

The germinal vesicle (the large nucleus of the oocyte) in the large ovarian oocytes of birds is transparent (Fig.5) in contrast to the white opaque structure of the surrounding cicatricula. PURKINJE (1830) describes for the first time the avian germinal vesicle and gives it the name of “vesicula germinativa”: Habet itaque cicatricula ovi ovarii partem specialem et sibi propriam, vesiculam sphaericam subcompressam, membranula tenerrima constantem, lympha propria, fors generatrice repletam (inde vesiculam germinativam appellaverim), in fossam cumulo albo mammaeformi e globules composito. (Eggs in the avian ovary present in their cicatricular region a special characteristic structure with a spherical, partly compressed aspect. It contains its own liquid which is always surrounded by a tinny membrane and it is localized in a mammiform mass of globules, it has the power to generate a future germ (sic), therefore I give it the name of germinal vesicle). Thus for Purkinje the germinal vesicle was the germ-generating-structure. By contrast for Von Baer (1828) it was not yet clear if the avian embryo was only formed from the disintegrated germinal vesicle together with the spermatozoa, or that also material from the surrounding cicatricula (ooplasm) plays a role in germ formation. He speaks from “Umbildung” i.e. a series of transformations from simple towards more complex. It was COSTE (1850) who described for the first time the cleavage furrows in the avian blastoderm, formed in the cicatricula shortly after fertilization. So he demonstrated that the oocytal ooplasm of the cicatricula also gives rise to the avian germ. The germinal vesicle in birds, reptiles and cephalopods is according to COSTE (1850) always included in a fine granular layer, which forms the substrate for the later blastoderm from which the embryo will develop. SCHWANN (1839), who developed the general cell theory, was the first to understand that the oocyte (unfertilized egg) of a mammal must be considered as one cell. Owing to their enormous volume he still considered the large oocytes in the ovary of birds as multicellular. HOYER in 1858 clearly observed that the yolk of large avian oocytes contained no cells. Thus large intraovarian avian oocytes are single giant cells.

In 1865 JOHAN MENDEL discovered the fundamental laws of genetics. However his study has been negated and ignored for more then half a century (KRUMBIEGEL, 1957). The reason seems to be that in that period; Darwin described and propounded the variability of species whilst the work of Mendel just demonstrated a constant previsible evolution (DE VEER, 1969). Chronologically we can distinguish three steps in the pairing mechanisms of animals, resulting in the formation of a germ: pairing of the chromosomes during meiosis in the germ cells of the gonads of the parents, copulation of the parents, pairing of the gametes (unfertilized eggs with spermatozoa) at the moment of fertilization. By progression of the cytological techniques, at the end of the 19th century, chromosomes in meiocytes (during oogenesis or spermatogenesis) were seen as most prominent structures. So VAN BENEDEN (1883) in the eggs of Ascaris megalocephala and VAN BAMBEKE (1885) found, that at the end of the meiotic division, only half of the number of chromosomes was found in the gametes (mature oocytes and spermatozoa). At the moment of pairing of the gametes (fertilization) the specific number of chromosomes is restored in the zygote. This demonstrated for the first time that both the father and the mother afford the same quantity of chromosomal material during the formation of the zygote, which gives rise to all the cells of the embryo. According to Van Beneden (University of Liège) and Van Bambeke (University of Ghent) this suggested that embryonic development is bound to chromosomal material charged with hereditary characteristics. However it is only in 1902 that SUTTON makes the link between the meiotic dissociation of chromosomes and the segregation of hereditary characteristics described by Mendel. The localization of genes on the chromosomes of Drosophila by MORGAN (1910) permitted further development of genetical knowledge. In recent decades the study of expression of genes during early developmental processes has become a powerful tool. Evolutionary and developmental biologists have joined forces to create a new field, unravelling the mysteries of evolution by studying the genes that control how an embryo develops (DEPEW & WEBER, 1995).

EXPERIMENTS ON EGGS

The theory of epigenesis, based on neoformation, as has been proposed by C.F. WOLFF since 1759, considers the egg not as a mosaic of territories with an achieved fate but as a progressive realization of stages which are conditioned by an earlier more simple stage (VAN SPEYBROECK et al; 2002). According to this epigenetic hypothesis the half of a germ can still give rise to a complete normal embryo of a smaller size. The egg is thus capable to regulate development of its parts. However in some species there is evidence for preformistic behaviour as the result of factors or territories present in the ooplasm. The link between ooplasmic structures (before fertilization) and structures in the germ (after fertilization) is not always obvious and differs from species to species. So the first experimental investigations in embryology by CHABRY (1887) and ROUX (1888) and CONKLIN (1905) suggested prelocalization – preformation mechanisms by mosaicism. Indeed in some species (ascidian or amphibian) an isolated hemisected egg (containing one of the two first blastomeres) will develop only in the corresponding half (left or right) of the embryo. The term mosaic development is used as originally defined by CONKLIN (1905) in ascidian species: each region of the whole fertilized egg would be able to form more or less independently on its own. The development of the entire embryo was regarded as being the sum of the development and interaction of the individual parts. Here the concept of “preformation” more particulary mosaicism no longer refers to the strong preformationistic theory described earlier in this review (pre-existence of parts), but as a soft preformation or preexistence, inclining to a more sophisticated version of preformation, due to ooplasmic determinants. OSCAR HERTWIG introduced in 1916 the concept of “preformed epigenesis”: the development of multicellular organisms from a fertilized egg is an epigenetical process whose species-specific course is firmly determined by the preformed hereditary substance which serves as its basis. In other species (echinoderms) an isolated half of an egg will produce a complete miniature embryo (DRIESCH, 1891) as the result of so-called regulation phenomena. This can however been explained by the all or not existence in the isolated halves or parts of all the different kinds of ooplasm in the blastomeres. Indeed during a study of the early embryonic avian development (CALLEBAUT, 1987), I found preformation evidence for the role of the four ooplasms (α, β, γ, δ) which present an onion-peel distribution in the oocytal germ disc region (Fig. 5). I demonstrated that α ooplasm plays a fundamental role during the cleavage stage by penetrating along with the cleavage furrows into the underlying ooplasms (Fig. 6). The β ooplasm originally mainly localized in the peripheral region of the area centralis becomes concentrated in the primitive streak by converging phenomena. The γooplasm finally gives rise to Rauber’s sickle. So for instance in the large blastomeres of the early avian germ disc the four fundamental ooplasms can still be observed as is the case in the whole younger germ (CALLEBAUT, 1987) (Figs 5; 6). That the caudocephalic axis of a vertebrate embryo is not preformed in the egg but can develop under influence of gravity was probably first observed by WINTREBERT (1922) in Selachians. Indeed, Wintrebert described the relationship between the spatial orientation of the early Selachian blastodisc on its egg yolk with respect to the vertical and the ensuing development of its caudo-cephalic axis. Also in the Selachian egg any part of the periphery of the blastodisc can give rise to the embryonic caudal edge: the only condition is that it should correspond temporally to its highest point. Still in the 20th century preformistic views concerning the craniocaudal axis of the future vertebrate embryo, persisted. So BARTELMEZ (1912; 1918) claimed that the craniocaudal axis of the future pigeon embryo was already established in the ovarian primordial follicle, visible as the long axis of the oocyte. According to VAKAET (1953; 1955) and FAUTREZ & VAKAET (1954) the plane of bilateral symmetry of the ovoviviparous teleostean fish, Lebistes reticulatus, is already predetermined in the oocyte during previtellogenesis. This view, is contested, however by CLAVERT & FILOGAMO (1957 a; b). Indeed, after fertilization the Lebistes reticulatus egg becomes movable within its follicle and undergoes an orienting rotation by gravity whereby the future germinal disc is turned to the upper pole of the egg. By contrast to vertebrates the cranio-caudal axis of the Drosophila embryo seems to be preformed by the localisation of ooplasmic determinants within the oocyte (ST. JOHNSTON & NÜSSLEIN-VOLHARD, 1992). Bicoid mRNA localised in the cranial pole is translated after fertilisation to give rise to a morphogen gradient of Bicoid protein that patterns the head and thorax (BERLETH et al., 1988). Similarly, oskar mRNA is localised into the caudal pole of the oocyte where it directs the assembly of the pole plasm, which contains the caudal and germ line determinants (EPHRUSSI & LEHMANN, 1992). J. BRACHET (1933; 1941; 1961) and CASPERSON (1941) discovered the role of RNAs in the synthesis of proteins. In 1953, Watson & Crick found the basis of our knowledge about the replication mechanism of DNA synthesis, giving rise to the development of molecular biology. Particularly in oocytes, intense synthesis of RNA’s and proteins takes place (BRACHET, 1941). These molecules (messenger RNAs) will in part pass into the ooplasm of the zygote after fertilization and will play a fundamental role during early embryonic development. According to the hypothesis of DALCQ & PASTEELS (1937) the original localization of the ooplasmic constituents in the egg, even not genetically determined, can influence early stages of embryonic development. Thus a kind of molecular preformism, present in the ooplasm, must be accepted. The term epigenetics was introduced by C.H. WADDINGTON (1940; 1952) over 60 years ago as the study of those processes involved in the unfolding of development. The discovery of the role of DNA in inheritance has cast a shadow over this discipline for decades (CAVALI, 2006). It was found that molecular machines act on the chromatin to regulate gene expression. These epigenetic regulators play a crucial role in the global shaping and maintenance of developmental patterning. Histone modifications seem to play a major regulatory role. Epigenetics has then been redefined as the study of heritable traits that are not dependent on the primary sequence of DNA (CAVALI, 2006). Ooplasm remains indispensable for embryonic development, since until now in the absence of ooplasm no cloning of individuals can be obtained. The transplantation of somatic nuclei into maturing amphibian oocytes by GURDON (1968; 1969) demonstrated the fundamental effect exerted by the ooplasm and eventually the nucleoplasm of the germinal vesicle on an implanted nucleus for further embryonic development.

Fig. 6. – Schematic representation of the localization of the ooplasms in the first large “closing” blastomeres; the forming subgerminal space is lined by a narrow α layer; F: cleavage furrow; NP: nucleus of Pander containing δ ooplasm (after CALLEBAUT, 1987).

PERSISTENCE OF FEMALE GERM LINE CELLS FROM EMBRYO TO ADULT

Until the years ’60 of the previous century numerous investigators claimed that the primitive sex cells in the ovary sooner or later all degenerate and completely disappear (FIRKET, 1914). Indeed a great part of the germ cells in the ovary degenerate and their investing follicular cells have a hormonal influence. Therefore it was thought that the definitive germ cells of the adult ovary develop secondarily by neoformation from somatic cells in the so-called germinal epithelium covering the ovary and develop epigenetically through transformation of the common coelomic epithelial cells (BLOOM & FAWCETT, 1962). This seems not to be the case. Indeed in the case of birds, by radioactive labelling with 3H-thymidine of the chromosomes of embryonic oocytes during the ultimate premeiotic DNA synthesis period (CALLEBAUT, 1967), I could demonstrate that the labelling persisted into the chromosomes of the adult oocytes. The adult oocytes are thus derived from the embryonic oocytes, present already long before hatching (CALLEBAUT, 1973). In mice similar results were obtained by PETERS et al. (1962), LIMA DE FARIA & BORUM (1962) and CRONE et al. (1965). This demonstrates that birds and mammals contain a final stock of oocytes, at the end of their embryonic development, which will be exhausted progressively during further life [part of the hypothesis of WALDEYER (1870)].

DISCOVERY
OF EMBRYONIC INDUCTION

In 1924, SPEMANN & MANGOLD describe the induction phenomena i.e. an embryonic structure functions as an organizing inductor on another structure (reactor) in its neighbourhood and modifies this reactor into a new structure without itself affording a cellular contribution to this new structure. SPEMANN & MANGOLD (1924) discovered the so-called “Spemann organizer” in amphibian embryos. They observed that after transplantation of a piece of dorsal blastopore lip of the early gastrula to the ventral side of another embryo, a secondary embryonic axis developed on this side. So the existence of induction phenomena between different associated parts of embryonic tissues demonstrated epigenetic development. The Spemann’s organizer redeterminates the fate of a part of the host cells and induced them to form axial structures, more particularly a central nervous system. The formation of a neural plate which gives rise to the central nervous system was considered to be the result of an inductive influence from the underlying mesoderm with organizer property.

Fig. 7. – Schematic representation of the components of an unincubated avian blastoderm seen from its deep side after removal of the subgerminal ooplasm, ready for culture; CRGW: cranial germ wall; CAGW: caudal germ wall; CMZ: caudal marginal zone; E: incomplete endophyll sheet; L: lacune in the deep layer; RS: Rauber’s sickle with its fragmentary sickle horns (SH) enclosing the area centralis. The vertical interrupted double line represents the future localization of the PS after incubation.

Fig. 8. – Schematic representation (simplified after CALLEBAUT, 1993b) of the localization of two intraoocytally radioactively labelled layers (respectively 4 or 6 days after a maternal radioactive injection) in the γ and δ ooplasms on a midsagittal section through an unincubated avian blastoderm (presenting a sickle-shaped bilateral symmetrization). Note the permanent boot-shaped deformation of the ooplasmic layers around the nucleus of Pander (NP) composed of δ ooplasm (white) and in the surrounding γ ooplasm (grey). The toe-shaped part of both γ and δ layers are expanded and horizontally flattened. They remain in the underlying ooplasm below the anti-sickle region (AS) and they have lost contact with the blastoderm. Caudally, in the heel-shaped part, the γ ooplasm is more condensed and adheres vertically to the upper layer, forming Rauber’s sickle (RS); the heel and middle part of the layers containing δ ooplasm of the nucleus of Pander (NP) are taken up in the caudal part of the area centralis of the blastoderm and later segregate progressively as endophyll (E) which also contains δ ooplasm (white) (CALLEBAUT, 1987; 1993a); the superficial upper layer is mainly composed of β

ooplasm (after CALLEBAUT, 2005).

OOPLASMIC DETERMINANTS
IN SOMATIC TISSUES

During the last decades it became clear that induction phenomena not only occur as the result of interaction between embryonic structures (in the sense of SPEMANN & MANGOLD, 1924) but also extraembryonic ooplasmic structures, all or not nucleized, play a fundamental role in early embryonic induction phenomena. Indeed in 1969 NIEUWKOOP discovered that during early blastulation in amphibians, signals are required from a region which is localized vegetal to the prospective dorsal blastopore lip, to initiate development of the mesoderm (Spemann’s organizer). This vegetal region is now designated as Nieuwkoop’s centre. In amphibians, Wnts seem to be the primary axis (formation of head and trunk-tail regions) inducing substances. Indeed in Xenopus, microinjection of several Wnts into the ventral cells of the early embryos leads to complete duplication of the body axis (CADIGAN & NUSSE, 1997; DEARDORFF et al., 1998). This duplication is believed to arise from the formation of a second Nieuwkoop’s centre (dorsal vegetal cells) of the early blastula which then induces overlying tissue in order to it might become the Spemann’s organizer (homologous to the avian primitive streak and nodus, WADDINGTON, 1932). Indeed in birds and mammals the properties of the Spemann’s organizer are performed by the node (Hensen’s node in birds) which is the rostral end of the full grown primitive streak. We found that in birds, RAUBER’s sickle (1876) (Figs 7; 8) has a homologous function as Nieuwkoop’s centre in amphibia. It functions indeed as the primary major organizer, initiating gastrulation phenomena and primitive streak formation (CALLEBAUT & VAN NUETEN, 1994; CALLEBAUT et al., 2003a). The localization and function of Rauber’s sickle present a strong similarity with the localization and function of the ascidian Wnt gene, Hr Wnt-5 from Halocynthia roretzi (SASAKURA et al., 1998). Indeed HrWnt-5mRNA is present in the vegetal cortex of unfertilized eggs. After fertilization, HrWnt-5 moves to the equatorial region to form a sickle-shaped structure after which this mRNA is concentrated in the most caudal region of the embryo. That Vg1 (the axis inducer) present in the avian Rauber’s sickle material (inclusive in the sickle horns) and not in the caudal marginal zone has recently been demonstrated (BERTOCCHINI & STERN, 2007). Hensen’s node must be considered as a secondary major organizer linked to Rauber’s sickle via a rostral outgrowth of the latter i.e. sickle endoblast. At the moment of the sickle-shaped bilateral symmetrization (characterized by the appearance of Rauber’s sickle), there oocurs a spatially oblique, sickle-shaped uptake of γ and δ ooplasms by oblique position in utero which become incorporated into the deeper part of the avian blastoderm (Fig. 8). This provokes an unidirectional chaos (radial symmetry breaking) in the ooplasm according to the principle of PRIGOGINE (PRIGOGINE & LEFEVER, 1968) which is indispensable for further development. These ooplasms seem to contain ooplasmic determinants (CALLEBAUT, 2005) which initiate (perhaps by Wnt signalling) either early gastrulation or neurulation phenomena by positional information (CALLEBAUT et al., 2003a). First we found (cytological) evidence for a radial predisposition (preformation) in the premature avian oocyte i.e. radially symmetric and concentric distribution of groups of mitochondria (CALLEBAUT, 1972) (Fig. 9). After the eccentric sickle shaped tilting of the yolk and ooplasmic layers in the fertilized egg by oblique positioning (CALLEBAUT et al., 2000), we observed the presence of predisposed sickle-shaped anlage fields in the upper layer (CALLEBAUT et al., 1996). There is a strong similarity with the gastrulation and neurulation phenomena described by VANDEBROEK (1969) in selachian germs (also developing on very large eggs). Indeed, in the latter vertebrate group there exist also analogous sickle-shaped anlage fields, localized in the upper layer in the same succession order as in birds. These fields are localized in the concavity of Rauber’s sickle in unincubated diblastic avian blastoderms which thus present a sickle-shaped bilateral symmetry (Fig. 10). Finally from this sickle-shaped bilateral symmetric disposition a primitive streak and neural plate will develop by convergent extension movements under influence of signalling molecules secreted by Rauber’s sickle (CALLEBAUT et al., 2003a) (Fig. 11) so a triblastic one-axis-containing embryo is formed. The early neural plate inducing structure which forms a deep part of the blastoderm is the δ ooplasm-containing endophyll (primary hypoblast) (Fig. 8). Together with the primordial germ cells it is derived from the superficial centro- caudal part of the nucleus of Pander which also contains δ ooplasm. It is indeed known that the prelaid nucleus of Pander (as is also the case for the endophyll) can induce the upper layer to form an early neural plate (CALLEBAUT et al., 2004b). The other structure (γ ooplasm) which is incorporated into the caudolateral deep part of the blastoderm forms Rauber’s sickle (Figs. 7; 8). It induces first gastrulation (intramuros) and later blood island and coelom formation (extramuros). Rauber’s sickle develops by ingrowth of blastodermal cells into the γ ooplasm (CALLEBAUT, 1994), which surrounds the nucleus of Pander (Fig. 8). The Rauber’s sickle constitutes the primary major organizer of the avian blastoderm. Rauber’s sickle generates only junctional and sickle endoblast and by positional information, organizes and dominates the whole blastoderm (first gastrulation, neurulation, and later coelom and cardiovascular system formation) (CALLEBAUT et al., 2003b). Fragments of the horns of Rauber’s sickle extend far cranially into the lateral quadrants of the unincubated blastoderm, so that often Rauber’s sickle material forms three quarters of a circle (Fig. 7). This explains the so called regulative capacities of isolated blastoderm parts with the exception of the anti-sickle region and/or the central blastoderm region, where no γ ooplasm and no Rauber’s sickle material is present (which again demonstrates the influence of the γ ooplasm) (CALLEBAUT, 2005). Recently we demonstrated that in the one and the same species (Gallus domesticus) and the same unincubated blastoderm both mosaic development or regulation phenomena can be obtained (CALLEBAUT et al., 2007). Indeed after hemi-sectioning of unincubated chicken blastoderms and culturing both halves formatted in vitro, two kinds of development can be discerned: 1. When the unincubated blastoderms were hemi-sectioned according to the plane of bilateral symmetry, going through the middle region of Rauber’s sickle (Fig. 12), we obtained two hemi-embryos (a left and a right one) containing each a half primitive streak (starting from the most median parts of Rauber’s sickle) giving rise to a half mesoblast mantle and a half area vasculosa, which differentiate incompletely thus indicating mosaic development. This (mediosagittal) hemi-sectioning of the avian blastoderm is comparable with the unilateral destruction experiments of the first two ascidian blastomeres by CHABRY (1887) and CONKLIN (1905). Indeed, the ascidian two-cell embryo already presents a left-right asymmetry visualized by the natural mediosagittal cleavage plane through the caudal sickle-shaped Wnt gene expressing zone (SASAKURA et al., 1998), homologous with Rauber’s sickle. 2. When the unincubated blastoderm is hemi-sectioned more obliquely going through a more lateral part of Rauber’s sickle (sickle horn), two complete bilaterally symmetrical miniature embryos will form indicating so-called regulation phenomena (Fig. 13). We demonstrated that those two types of development are in reality due to the different spreading and concentration of Rauber’s sickle tissue around the area centralis (CALLEBAUT et al, 2007). Embryonic regulation must thus not be considered as a kind of totipotent regeneration capacity of isolated parts of the unincubated avian blastoderm but depends on the spatial distribution of a kind of extraembryonic tissue (Rauber’s sickle) build up by the late oblique uptake of γ ooplasm at the moment of bilateral symmetrization (CALLEBAUT, 1994; CALLEBAUT et al., 2000) forming an ooplasmic mosaic. Thus not only embryonic gene expression phenomena take place during early development but also uptake of extraembryonic preformed ooplasmic determinants play a fundamental role for the initiation of gastrulation, neurulation and cardiovascular development. So finally three ooplasms are respectively found in the three elementary tissues (not germ layers !) of the early, unincubated avian blastoderm (Figs 7; 8): the upper layer from which the embryo proper develops containing mainly β ooplasm (forming the embryonic stem cells), the Rauber’s sickle containing γ ooplasm and the endophyll (primary hypoblast) containing δ ooplasm both forming the extraembryonic tissues. In mammals also the embryonic ectoderm (upper layer) constitute the real stem cells which under influence of the surrounding extraembryonic tissue transform in mesendodermal and neural cell lines (HADJANTONAKIS & PAPAIOANNAOU, 2001). So the general body plan (bilaterally symmetric) is established in the diblastic germ. This confirms, in the case of birds, the existence of a similar master plan for the early development as was proposed for all chordates by EYAL-GILADI (1997). According to the hypothesis of Eyal-Giladi, the speed at which axialization of the embryo proper takes place, depends on the translocation speed of oocytal determinants from the vegetal pole towards the future dorsocaudal side of the embryo. After arrival at their destination, the activated determinants form in all chordates, an induction center homologous to the amphibian “Nieuwkoop center” and the ascidian Wnt expressing sickle shaped region, which later organize the formation of the intraembryonic “Spemann’s organizer”. Thus in birds by using radioactive or trypanblue induced fluorescence oocyte labelling we could demonstrate that a kind of evolutive preformism exists which follows more or less the main evolution of the animal kingdom: from a radial symmetric disposition in the premature oocyte (as exists in coelenterates) via a sickle-shaped diblastic bilateral symmetric germ to a triblastic one-axis-containing embryo (gastrulation induced in aves by Rauber’s sickle). Finally, we observed the formation of the coelomic cavity with associated cardiovascular system (typical for coelomates; DOLLANDER and FENART, 1973) also induced by Rauber’s sickle material (CALLEBAUT et al., 2002; 2004a). An ooplasmic influence on spatial patterning of the mouse blastocyst has been demonstrated by GARDNER (1997), by using the localization of the polar body as a marker for the animal pole. Also PIOTROWSKA and ZERNICKA-GOETZ (2001) showed that the sperm entry position products the plane of initial cleavage of the mouse egg and can define embryonic and abembryonic halves of the future blastocyst. In addition, the cell inheriting the sperm entry position acquires a division advantage and tends to cleave ahead of its sister.

Fig. 10. – Schematic representation of the mean localization of the predisposed (not definitively committed) anlage fields (in good order but with possible partial overlapping of neighbouring parts) in the upper layer of the area centralis of a chicken (Gallus domesticus) unincubated blastoderm (slightly simplified after CALLEBAUT et al., 1996). Note the general eccentric sickle-shaped aspect of the anlage fields in the area centralis after the radially symmetry breaking eccentricity of the subgerminal ooplasms. There is an obvious parallelism between the sickle shape of the anlage fields in the UL and the ovoid central sub-germinal ooplasmic layers (CALLEBAUT et al., 2000). The curved arrows on the anlage fields indicate the logically previsible converging movements of the upper layer during the ensuing gastrulation (WETZEL, 1929) and neurulation (BORTIER and VAKAET, 1992).

Fig. 11. – Combined schematic drawing representing: 1. the hypothetical diffusion of morphogens or signalling molecules indicated by small dots) emanating from Rauber’s sickle (RS) and its sickle horns (SH) into the neighboring tissues of the avian blastoderm, i.e., into the area centralis (AREA CENTR), into the caudal marginal zone (CMZ) and into the caudal germ wall (CGW), where they can influence (induce or inhibit) ectopically placed structures (endophyll, Rauber’s sickle fragments, sickle or junctional endoblast) to form or not to form a second streak; 2. the broad movements (indicated by curved arrows) of cell groups in the upper layer of the area centralis, in the direction of the median primitive streak (partially after WETZEL, 1929). The curved legs of the U-shaped lines indicate moving fronts of cell groups (corresponding to local temporal primitive streak anlagen) that will ingress after fusion into the final median primitive streak.

Fig. 12. – Schematic representation of the hemi-sectioning through the middle of the sickle of Rauber (double lined) of an unincubated blastoderm; both halves of the sectioned blastoderm are represented at some distance (ready for culture); SH: sickle horns of Rauber’s sickle extending far cranially; R: right half blastoderm and L: left half blastoderm will each transform in a right and left half embryo, indicating mosaicism; the thick half arrows represent the formation of hemi-primitive streaks at the cut edge of the blastoderm after incubation.

Fig. 13. – Schematic representation of the oblique hemi-sectioning through the lateral part of the Rauber’s sickle of an unincubated avian blastoderm; the arrows 1 and 2 indicate the place where after culture a bilateral symmetric primitive streak will appear respectively under the influence of the middle part of the Rauber’s sickle (double lined) or under influence of the remaining fragmentary sickle horn (SH), indicating regulation.

OOPLASMIC DETERMINANTS
IN PRIMORDIAL GERM CELLS

NUSSBAUM (1880; 1901) and WEISMANN (1892) were the first to propose the preformation thesis in birds i.e. that the primordial germ cells have inherited and retained for long periods the yolk from the precursor oocyte from which their egg yolk ball and blastoderm are derived. In birds DANTSCHAKOFF (1908) observed so called “endodermal Wander Zellen“ in the space between the upper layer (epiblast) and deep layer. The primordial germ cells (PGC’s) are indeed only first unequivocally distinguishable in the 1-8 somite chicken blastoderm between epiblast and endophyll of the germinal crescent (SWIFT, 1914). BOUNOURE (1939) describes a kind of “germ plasm” in the vegetal pole of Anura eggs. By irradiation this germ plasm could be inactivated, giving rise to sterile individuals. Only cells which inherited part of this germ plasm gave rise to PGC’s. It was thus the kind of ooplasm which determined the fate of the involved cells. The germ plasm thus contains so-called “Keimbahn determinants”. Because PGC’s in birds are found close to the endophyll and seem to immerge from it, VAKAET (1962) supposed that they were derived from the endophyll. DUBOIS (1967; 1969) also concluded that the endophyll is at the origin of the formation of PGC’s in birds. The precursors of the PGC’s which are often initially morphologically indistinguishable from the surrounding somatic cells in earlier stages, are called presumptive primordial germ cells (p PGC’s). These divide mitotically to produce one PGC containing “Keimbahn determinants” and one somatic cell. Thus in general it was accepted that chicken germ cells originate from the primitive deep layer. By contrast, EYAL-GILADI et al., (1981) concluded by using chick-quail chimeras, made before primitive streak formation

(i.e. stage XIII: 10-12h incubation), that avian PGC’s

were from epiblastic origin. Avian PGC’s were then thought to arise through a gradual epigenetic process. However, in these older blastoderms the deep layer is no longer composed of endophyll but mainly formed by sickle endoblast, derived from Rauber’s sickle (CALLEBAUT et al., 1997). Indeed the endophyll and associated PGC’s are then already displaced cranially and adhere to the deep cranial part of the epiblast and to the there present hemicircular fibrous bands (ENGLAND, 1983). They will form part of the endophyllic crescent in older stages. The experiments of CUMINGE and DUBOIS (1992) seemed to confirm the thesis of Eyal-Giladi et al., but they also investigated similar old blastoderm stages which greatly differ from the unincubated blastoderm. By using trypan blue induced fluorescent labelling of the ooplasmic yolk layers of quail oocytes during their final post-lampbrush stage, I could demonstrate that primordial germ cells together with the endophyll contain yolk from the deep central region of the germ disc i.e. δ ooplasm from the superficial part of the nucleus of Pander (CALLEBAUT, 1984; 1987). So nearly 95% of the PGC’s can be labeled 6-7 days after one single injection of trypan blue to the mother quail. Oocytal yolk labeling, 1 to 4 days after an injection gives no labeling of the primordial germ cell yolk, but gives labeling of more superficial somatic cells which contain more superficial ooplasms (β or γ). The observed trypan blue induced fluorescent yolk labelling in the caudally in the area centralis localized endophyll of the unincubated quail blastoderm (CALLEBAUT, 1987) is in agreement with the observed localization of the pPG cells (also containing δ yolk) after transsection experiments (FARGEIX, 1967; ROGULSKA, 1968; DUBOIS and CROISSILLE, 1970) i.e. mainly in the caudal region of the unincubated blastoderm. The original deep and central localization of pPGC material has recently been confirmed by the use of a chicken vasa homologue (TSUNEKAWA et al., 2000). Chicken vasa protein forms part of the mitochondrial cloud in younger chick oocytes and localizes to the central cleavage furrows (which extend into the δ ooplasm of the nucleus of Pander) until stage IV (EYAL-GILADI and KOCHAV, 1976). At that moment 6 to 8 cells of the approximately 300 blastomeres containing germ, present vasa protein and are probably p PGC’s. (TSUNEKAWA et al., 2000). The data of CALLEBAUT (1984) and TSUNEKAWA et al. (2000) thus indicate that a kind of deep preformation may be the mechanism for germ cell specification in birds. As in Xenopus, in the quail there are two known populations of oocytal mitochondria which become finally localized in the early embryo: one population becomes localized in the vegetal pole where it forms a component of the germ plasm in Xenopus (MIGNOTTE et al., 1987; TOURTE et al., 1984) and a component of the nucleus of Pander (δ ooplasm) in the quail (CALLEBAUT, 1984; D’HERDE et al., 1995). The other population of mitochondria is localized much more superficially and forms the obvious radially and concentrically disposed group around the germinal vesicle both in Xenopus (MIGNOTTE et al., 1987) as in the quail (Ticos: CALLEBAUT, 1972; 1983; D’HERDE et al., 1995). These mitochondria will populate the somatic tissues of the offspring (in Xenopus: DAWID and BLACKLER, 1972) (in quail: WATANABE et al., 1985). Our conclusion is in agreement with WOLPERT (1998) and EXTAVOUR and AKAM (2003) that epigenetic germ cell development (derived from somatic stem cells) is an exception and that most animals use localized ooplasmic determinants to specify the germ line.

ACKNOWLEDGEMENTS

The author thanks very much Mr F. De Bruyn and Mrs.

E. Goeman for artwork, Mrs E. Van Nueten for excellent technical assistance and Mrs V. De Maere and Mr H. Callebaut for careful typing the manuscript. The author is also very grateful to Prof. Dr. H. Bortier and to Prof. Dr. F. Harrisson for the possibility to work in their laboratory.

REFERENCES

ADELMANN H (1942). The embryological treatises of Hieronymus Fabricus ab Aquapendente. Cornell Univ Press Ithaca, New York.

ADELMANN H (1966). Marcello Malpighi and the evolution of embryology. 5 vols Cornell Univ Press Ithaca New York.

ARISTOTELES (4th cent B.C.) Historia animalium (cited by THOMPSON, 1910).

AVERROES (12th cent A.C.) cited by DOLLANDER & FENART (1973).

BARTELMEZ GW (1912). The bilaterality of the pigeon’s egg I. A study in egg organization from the first growth period of the oocyte to the beginning of cleavage. Journal of Morphology,

23: 269-328.

BARTELMEZ GW (1918). The relation of the embryo to the principal axis of symmetry in the bird’s egg. Biological Bulletin,

35: 319-361.

BERLETH T, BURRI M, THOMA G, BOPP D, RICHSTEIN S, FRIGERIO G, NOLL M & NUSSLEIN-VOLHARD C (1988). The role of localization of bicoid RNA in organizing the anterior pattern of the Drosophila embryo. The European Molecular Biology Organization Journal, 7: 1749-1756.

BERTOCCHINI F & STERN CD (2007). Gata2 as a putative inhibitor of axis formation in the anterior part of the early chick embryo. The chick as a model organism: genes, development and function. International chick meeting Barcelona.

BISCHOFF (1842) cited by LAMS (1935).

BLOOM W & FAWCETT DW (1962). A textbook of Histology. Saunders Company Philadelphia, London.

BLUMENBACH (1789) cited by LAMS (1935).

BONNET C (1740) cited by ROSTAND and TÉTRY (1962).

BORTIER H & VAKAET L (1992). Fate mapping the neural plate and the intraembryonic mesoblast in the upper layer of the chicken blastoderm with xenografting and time-lapse videography. Development Supplement: 93-97.

BOUNOURE L (1939). L’origine des cellules reproductrices et le problème de la lignée germinale. Collection des actualités Paris.

BRACHET J (1933). Recherches sur la synthése de l’acide thymonucléique pendant le développement de l’œuf d’oursin. Archives Biologie, 44: 519-576.

BRACHET J (1941). La localisation des acides pentonucléiques dans les tissues animaux et les œufs d’amphibiens en voie de développement. Archives Biologie, 53: 207-257.

BRACHET J (1961). The role of ribonucleic acid and sulfhydril groups in morphogenesis. In: MX Zarrow Basic Books (ed), Growth in Living Systems, New York.

CADIGAN K & NUSSE R (1997). Wnt signalling, a common theme in animal development. Genes Development, 11: 3286-3310.

CALLEBAUT M (1967). Premeiosis and premeiotic DNA synthesis in the left ovary of the female chick embryo. Journal Embryology Experimental Morphology, 18: 299-304.

CALLEBAUT M (1972). Cytoplasmic uniaxial radial symmetry during the early final growth period in the oocytes of the Japanese quail (Coturnix coturnix japonica). Experientia, 28: 62-63.

CALLEBAUT M (1973). Correlation between germinal vesicle and oocyte development in the adult Japanese quail (Coturnix coturnix japonica): A cytochemical and autoradiographic study. Journal Embryology Experimental Morphology, 29: 145-157.

CALLEBAUT M (1975). Bijdrage tot de studie van de oogenesis van de vogels. PhD thesis, RUCA edit.

CALLEBAUT M (1983). Electron microscope study of Ticos (³Hthymidine incorporating cytoplasmic organelles) in the germinal disc of the post-lampbrush oocytes of Japanese quail. International Research Communication System Medical Sciences, 11: 491-492.

CALLEBAUT M (1984). Avian primordial germ cells contain yolk from the nucleus of Pander. International Research Communication System Medical Sciences, 12: 730-731.

CALLEBAUT M (1987). Ooplasmic localization and segregation in quail germs: fate of the four ooplasms. Archives Biologie (Bruxelles), 98: 441-473.

CALLEBAUT M (1993a). Early eccentricity in gravitationally oriented quail germs. European Journal Morphology, 31: 5-8.

CALLEBAUT M (1993b). Unequal caudocephalic ooplasmic uptake and eccentric formation of the subgerminal space below unincubated quail blastoderms presenting a Koller’s sickle. Belgian Journal Zoology, 123: 107-112.

CALLEBAUT M (1994). Relationship between the avian blastoderm and the subgerminal ooplasm. European Archives Biology, 105: 111-123.

CALLEBAUT M (2005). Origin, fate and function of the components of the avian germ disc region and early blastoderm: role of ooplasmic determinants. Developmental Dynamics,

233: 1194-1216.

CALLEBAUT M & VAN NUETEN E (1994). Rauber’s (Koller’s) sickle the early gastrulation organizer of the avian blastoderm. European Journal Morphology, 32: 35-48.

CALLEBAUT M, VAN NUETEN E, BORTIER H, HARRISSON F & VAN NASSAUW L (1996): Map of the Anlage fields in the avian unincubated blastoderm. European Journal Morphology, 34(5): 347-361.

CALLEBAUT M, VAN NUETEN E, BORTIER H, HARRISSON F, VAN NASSAUW L & SCHREVENS A (1997): Spatial relationship between endophyll, primordial germ cells, sickle endoblast and upper layer in cultured avian blastoderms. Reproduction Nutrition Development, 35: 293-304.

CALLEBAUT M, VAN NUETEN E, HARRISSON F & BORTIER H (2000). Mechanisms of caudocephalic axis formation in the avian germ. Belgian Journal Zoology, 130: 67-79.

CALLEBAUT M, VAN NUETEN E, BORTIER H & HARRISSON F (2002). In the absence of Rauber’s sickle material, no blood islands are formed in the avian blastoderm. Journal of Morphology, 253: 132-147.

CALLEBAUT M, VAN NUETEN E, BORTIER H & HARRISSON F (2003a). Positional information by Rauber’s sickle and a new look at the mechanisms of primitive streak initiation in avian blastoderms. Journal of Morphology, 255: 315-327.

CALLEBAUT M, VAN NUETEN E, BORTIER H & HARRISSON F (2003b).Rauber’s sickle generates only extraembryonic tissues (junctional- and sickle endoblast), and, by positional information information, organizes and dominates the whole avian blastoderm (gastrulation, neurulation and blood island formation). Belgian Journal Zoology, 133(1): 45-59.

CALLEBAUT M, VAN NUETEN E, BORTIER H & HARRISSON F (2004a). Induction of the avian coelom with associated vitelline blood circulation by Rauber’s sickle derived junctional endoblast and its fundamental role in Heart formation. Journal of Morphology, 259: 21-32.

CALLEBAUT M, VAN NUETEN E, BORTIER H & HARRISSON F (2004b). Induction and improved embryonic development by the nucleus of Pander in associated avian blastoderm parts: influence of δ or γ ooplasm. Journal of Morphology,

260: 201-208.

CALLEBAUT M, VAN NUETEN E, HARRISSON F & BORTIER H (2007). Mosaic versus regulation development in avian blastoderms depends on the spatial distribution of Rauber’s sickle material. Journal of Morphology, 268: 614-623.

CAVALLI G (2006). Chromatin and epigenetics in development: blending cellular memory with cell fate plasticity. Development, 133: 2089-2094.

CAULLERY M (1957). L’Embryologie. Collection “Que sais-je”. Presses Universitaires de France.

CASPERSON T (1941). Studien über den Eiweiszumsatz der Zelle. Naturwissenschaften, 29: 33-43.

CHABRY L (1887). Contribution à l’embryologie normale et tératologique des Ascidies simples. Journal of Anatomy and Physiology, 23: 167-177.

CLAVERT J & FILOGAMO G (1957a). Sur la position du noyau de l’ovocyte et du plan de symétrie de l’embryon de Lebistes reticulatus. Comptes Rendus de la Societe Biologique (Paris), 151: 1740.

CLAVERT J & FILOGAMO G (1957b). Existence, chez Lebistes reticulatus d’une rotation d’orientation de l’œuf. Comptes Rendus de la Societe Biologique (Paris), 151: 2187.

CONKLIN E (1905). Mosaic development in ascidian eggs. Journal of Experimental Zoology, 10: 393-410.

COSTE M (1850). Recherches sur la segmentation de la cicatricule chez les oiseaux les reptiles écailleux, et poissons cartilagineux. Les Comptes rendus de l’Académie des Sciences,

30: 638-642.

CUMINGE D & DUBOIS R (1992). L’origine embryologique de la lignée germinale des oiseaux: étude expérimentale chez le poulet. European Archives of Biology, 103: 195-209.

CRUIKSHANK W (1797). Experiments in chick, on the third day of impregnation; the ova of rabbits were found in the fallopian tubes and on the fourth day after impregnation in the uterus itself: with the first appearances of the fœtus. Philosophical Transactions of the Royal Society of London, 87: 197-214.

CRONE M, LEVY E & PETERS H (1965). The duration of premeiotic DNA synthesis in mouse oocytes. Experimental Cell Research, 39: 678-688.

DALCQ A & PASTEELS J (1937). Une conception nouvelle des bases physiologiques de la morphogénése. Archives Biologie, 48: 669-710.

DANTSCHAKOFF W (1908). Entwicklung des Blutes bei den

Vögeln. Anatomische Hefte Abt 1, 37: 471-589.

DARWIN (1859) cited by LAMS (1935).

DAWID I & BLACKLER A (1972). Maternal and cytoplasmic inheritance of mitochondrial DNA in Xenopus. Developmental Biology, 29: 152-161.

DE GRAAF R (1672). De mulierum organis generationi inservientibus Hack, Leyden.

DEPEW D & WEBER B (1995). Darwinism evolving: system dynamics and the genealogy of natural selection. London: A Bradford Book, The MIT Press.

DE VEER P (1969). Leven en Werk van Hugo DeVries. Wolters-Noordhoff, Groningen.

DEARDORFF M, TAN C, CONARD L & KLEIN P (1998). Frizzled-8 is expressed in the Spemann organizer and plays a role in early morphogenesis. Development, 125: 2687-2700.

D’HERDE K, CALLEBAUT M, ROELS F, DE PREST B & VAN NASSAUW L (1995). Homology between mitochondriogenesis in the avian and amphibian oocyte. Reproduction Nutrition Development, 35: 305-311.

DOLLANDER A & FENART R (1973). Eléments d’embryologie (368 pp) seconde edit. Flammarion, Paris.

DRIESCH H (1891). Entwicklungsmechanische Studien I Der Werth der beiden ersten Furschungszellen in der Echinodermenentwicklung. Zeitschrift für Wissenschaftliche Zoologie,

53: 160-178.

DUBOIS R (1967). Localisation et migration des cellules germinales du blastoderme non incubé de poulet après les resultats de culture in vitro. Archives Anatomie Microscopique et Morphologie Experimentale, 56: 245-264.

DUBOIS R (1969). Données nouvelles sur la localisation des cellules germinales primordiales dans le germe non incubé de poulet. Les Comptes rendus de l’Académie des Sciences,

269: 205-208.

DUBOIS R & CROISILLE Y (1970). Germ-cell line and sexual differentiation in birds. Philosophical Transactions of the Royal Society of London, B259: 73-89.

ENGLAND M (1983). The migration of primordial germ cells in avian embryos. Current problems in germ cell differentiation 90-113. Symposium of British Society for developmental Biology Cambridge University press Cambridge Edit Anne McLaren and C C Wyhie.

EPHRUSSI A & LEHMANN R (1992). Induction of germ cell formation by oskar. Nature, 358: 387-392.

EXTAVOUR G & AKAM M (2003). Mechanisms of germ cell specification across the metazoans: epigenesis and preformation. Development, 130: 5869-5884.

EYAL-GILADI H (1997) Establishment of the axis in chordates: facts and speculations. Development, 124: 2285-2296.

EYAL-GILADI H & KOCHAV S (1976). From cleavage to primitive streak formation: a complementary normal table and a new look at the start stages of development of the chick. I General morphology. Developmental Biology, 49: 321-337.

EYAL-GILADI H, GINSBURG M & FARBAROV A (1981). Avian primordial germ cells are of epiblastic origin. Journal Embryology Experimental Morphology, 65: 139-147.

FABRICIUS AB AQUAPENDENTE A (1637). Opera Omnia anatomica et physiologica. lugduni batavorum typis, Isacus van der Mijn.

FARGEIX N (1967). Les cellules germinales chez de jeunes embryons de Canard dévelopés à partir de moitiés antérieures et postérieures de blastoderme. Comples Rendus Hebdomadaires de l’academie des Sciences Paris, 264: 2922-2925.

FAUTREZ J & VAKAET L (1954). Sur la persistence au cours du développement embryonnaire d’un plan de symétrie bilaterale, dans l’oocyte de premier ordre chez Lebistes reticulatus. Comptes Rendus de l’association des Anatomistes C417 (mentioned by RAVEN, 1961).

FIRKET J (1914). Recherches sur l’organogénése des glandes sexuelles chez les oiseaux. Archives Biologie Paris, 29: 201

351. GALENUS C cited by LAMS (1935). GARDNER RL (1997). The early blastocyst is bilaterally symmet

rical and its axis of symmetry is aligned with the animal-vegetal axis of the zygote in the mouse. Development, 124: 289-301.

GURDON JB (1968). Changes in somatic cell nuclei inserted into growing and maturing amphibian oocytes. Journal of Embryology Experimental Morphology, 20: 401-414.

GURDON JB (1969). The developmental capacity of nuclei taken from differentiating endoderm cells of Xenopus laevis. Journal of Embryology Experimental Morphology, 8: 505-526.

HADJANTONAKIS A & PAPAIOANNAOU V (2001). The stem cells

of early embryos. Differentiation, 68: 159-166. HAECKEL cited by HAMM and cited by LAMS (1935). HAECKEL (1868) cited by DOLLANDER & FENART (1978).

HARTSOEKER N (1694). Essay de Dioptrique, Paris. HARVEY W (1653). Exercitationes de generatione animalium. Londini. HERTWIG O (1916). Das Werden der organismen. Gustaf Fischer

Jena. JACOBI cited by LAMS (1935). JANSSEN cited by LAMS (1935). HOYER H (1858). Ueber die Eifollikel der Vögel, namentlich der

Tauben und Hühner. Müller’s Archif, 52: 52-60.

KRUMBIEGEL I (1957). Gregor Mendel und das Schicksal seiner Entdeckung. Wissenschaftliche Verlagsgesellschaft, Stuttgart.

LA BERGE A (1999). The history of science and the history of microscopy. Perspectives on Science, vol 7 n1: 111-142. L’ALLEMAND (1841). Sur les zoospermes. Anales des Sciences

Naturelles. Cited by LAMS 1935. LAMARCK (1802) cited by LAMS (1935). LAMS H (1935). De embryologie door de eeuwen heen.

Vlaamsch Diergeneeskundig Tijdschrift, Gent: 1-32. LINDEBOOM GA (1973). Regnier De Graaf: Leven en Werken 30/7/1641 - 17/8/1673 Edit Elmar BV Delft. LIMA DE FARIA A & BORUM K (1962). The period of DNA synthesis prior to meiosis in the mouse. Journal of Cell Biology,

14: 418-426. LÜTHY C (1996). Atomism, Lynceus and the fate of Seventeethcentury microscopy. Early Science and medicine, 1: 1-27. MALPIGHI M (1669). De Formatione pulli “in ovo”. dissertatione

epistolica. Cited by ADELMANN (1966). MAUPERTIUS (1745) cited by DOLLANDER & FENART (1973). MENDEL GJ (1865). Verhandlungen des naturforschenden

Vereins in Brünn.

MIGNOTTE F, TOURTE M & MOUNOLOU J (1987). Segregation of mitochondria in the cytoplasm of Xenopus vitellogenic oocytes. Biology of the Cell, 60: 97-102.

MIKHAILOV A (1997). Epigenesis versus preformation: first chapter of the Russian embryological research. International Journal of Developmental Biology, 41: 755-762.

MÜLLER (1844) cited by LAMS (1935).

NEEDHAM J (1959). A history of embryology, second edition, revised by A. HUGHES, Cambridge. University Press, Cambridge.

NIEUWKOOP P (1969). The formation of the mesoderm in urodelean amphibians. I Induction by the endoderm. Roux Archives of Developmental Biology, 162: 341-373.

NUSSBAUM M (1880). Zur Differenzierung des Geschlechts im Thierreich. Archives of Mikroskopie Anatomie, 18: 1-21.

NUSSBAUM M (1901). Zur Entwicklung des Geslechts beim Hühn. Verhandlungen der Anatomischen Gesellschaft, 75: 38-40.

PANDER C (1817). Historiam metamorphoseos quam ovum incubatum prioribus quinque diebus subit. Wirceburgi: FE Nitribitt. (69 pp).

PETERS H, LEVY E & CRONE M (1962). Desoxyribonucleic acid synthesis in oocytes of mouse embryos. Nature, 195: 915.

PIOTROWSKA K & ZERNICKA-GOETZ M (2001). Role for sperm in spatial patterning of the early mouse embryo. Nature, 409: 517-521.

PREVOST & DUMAS (1824) cited by LAMS (1935). PRIGOGINE I & LEFEVER R (1968). Symmetry breaking instabilities in a dissipative system II. Journal of Chemical Physics,

48: 1695-1700. PURKINJE JE (1830). Symbolae ad ovi avium historiam ante incubationem. Lipsiae, Leopold Vossii Bratislavia. RAUBER A (1876). Über die Stellung des Hühnchens im Entwicklungsplan. Leipzig W. Engelsmann.

RAVEN C (1961). Oogenesis: The storage of developmental information. Pergamon press, Oxford London New York Paris.

REAUMUR R & BEGUELIN F (1749). L’art de faire éclore et d’éléver en toute saison les oiseaux domestiques de toutes espèces, soit par le moyen de la chaleur du fumier, soit par le moyen de celle du feu ordinaire. Paris Vol.1: 364pp.

ROE SA (1981). Matter, life and generation. Eight tenth-century embryology and the HALLER-WOLFF debate. Cambridge University Press. Cambridge, London.

ROSTAND J & TÉTRY A (1962): “La vie” Larousse Paris.

ROGULSKA T (1968). Primordial germ cells in normal and transected duck blastoderms. Journal of Embryology and Experimental Morphology, 20: 247-260.

ROUX W (1888). Zur Frage der Axenbestimmung des embryo in Froschei. Biologisch Zeitblatt, 8: 399-413.

SANDER K (1996). On the causation of animal morphogenesis; concepts of German-speaking authors from Theodor Schwann (1839) to Richard Goldschmidt (1927). International Journal of Developmental Biology, 40: 7-20.

SASAKURA Y, OGASAWARA M & MAKABE W (1998). HrWnt-5: a maternally expressed ascidian Wnt gene with posterior localization in early embryos. International Journal of Developmental Biology, 42: 573-579.

SAWN CT (1997). Regnier de Graaf and the Graafian follicle. Historical note. The Endocrinologist, 7: 415-421.

SCHWANN T (1839). Mikroskopische Untersuchungen über dieÜbereinstimmung in der Struktur und dem Wachsthum der Thiere und Pflanzen. Berlin.

SHORT R (1977). The discovery of the ovaries. In: ZUCKERMANN I & WEIR (eds), “The ovary” , Second edition.

SPALLANZANI L (1775). De fenomeni della circulazione. Modena.

SPEMANN H & MANGOLD H (1924). Über die Induktion von Embryoanlagen durch Implantation artfremder Organisatoren. Roux Archives Entwicklungsmechanik, 100: 599-638.

ST JOHNSTON D & NÜSSLEIN- VOLHARD C (1992). The origin of pattern and polarity in the Drosophila embryo. Cell, 68: 201

219. SUTTON (1902) cited by LAMS (1935). SWIFT C (1914). Origin and early history of the primordial germ

cells in the chick. American Journal of Anatomy, 15: 483

516.

THOMPSON DAW (1910). “The works of Aristoteles translated into English. Vol IV” Historia animalium” Oxford Univ Press (Clarendon) London, New York.

THURET (1854) cited by CAULLERY.

TOURTE M, MIGNOTTE F & MOUNOLOU J (1984). Heterogeneous distribution and replication activity of mitochondria in X. laevis oocytes. European Journal of Cell Biology, 34: 171

178.

TREDERN DE LÉZÉREC LS (1808). Dissertatio inauguralis medica sistens ovi avium historiae et incubationis prodromum. Etzdorf, Jena.

TRIMBLEY cited by LAMS (1935).

TSUNEKAWA N, NAITO M, SAKAI Y, NISHIDA T & NOCE T (2000). Isolation of chicken vasa homolog gene and tracing the origin of primordial germ cells. Development, 127: 2741-2750.

VAKAET L (1953). Sur la symétrie de l’oocyte et la symétrie définitive chez Lebistes reticulatus. Comptes Rendus de la Societé de Biologie, 147: 531-533.

VAKAET L (1955). Recherches cytologiques et cytochimiques sur l’organisation de l’oocyte I de Lebistes reticulatus. Archives Biologie, 66: 1-71.

VAKAET L (1962). Pregastrulatie en gastrulatie der Vogelkiem. Ph. D. Thesis University of Ghent (Arsia) Ghent, Belgium. VAKAET L (1965). Karl-Ernst von Baer en het begin der weten

schappelijke embryologie. Scientiarum Historia, 7: 137-142. VAN BAMBEKE F (1870). Recherches sur le développement du Pélobate brun. Mémoires Académie Royale, 34: 1-66.

VAN BENEDEN E (1870). Recherches sur la composition et la signification de l’œuf des Oiseaux. Mémoires Académie Royale, 34: 186-210.

VANDEBROEK G (1969). Evolution des vertébrés de leur origine à l’homme. Paris, Masson.

VAN LEEUWENHOEK A (1678). Observationes de Anthony van Leeuwenhoek de natis e semine genitali animalculis. Letter dated November 1677. Philosophical Transactions of the Royal Society of London, 12: 1040.

VAN LEEUWENHOEK A (1683). An abstract of a letter from Mr Anthony Van Leeuwenhoek of Delft about generation by an animalcule of the male seed. Animals in the seed of a frog. Philosophical Transactions of the Royal Society of London,

13: 347-355.

VAN SPEYBROECK L, VAN DE VIJVER G & VANDEWAELE A (eds) (2002). From epigenesis to epigenetics. The Genome in context. Annals of the New York Academy of Sciences Vol. 981.

VAN SPEYBROECK L, DEWAELE D & VAN DE VIJVER G (2002). Theories in early embryology. Close connections between epigenesis, preformationism and self-organization. Annals of the New York Academy of Sciences, 981: 7-49.

VON BAER KE (1827). De ovi mammalium et hominis genesi. Lipsiae.

WADDINGTON CH (1932). Experiments on the development of chick and duck embryos, cultivated in vitro. Philosophical Transactions, B221: 179-230.

WADDINGTON CH (1940). Organizers and genes. University Press Cambridge. WADDINGTON CH (1952). The epigenetics of birds. University of

Press Cambridge. WALDEYER W (1870). Eierstock und Ei. Leipzig, W. Engelmann WATANABE T, MIZUTANI M, WAKAMA S & TOMITA T (1985).

Demonstration of maternal inheritance of mitochontrial DNA in chick-quail hybrids. Journal of Experimental Zell,

236: 245-247. WEISMANN A (1892). Das Keimplasm: eine Theorie der Vererbung. Fischer Iena.

WETZEL R (1929). Untersuchungen am Hühnchens. Die Entwicklung des Keims während der ersten beiden Bruttage. Roux Archives, 119: 188-321.

WINTREBERT P (1922). Polarité mécanique du germe des Sélaciens au temps de la gastrulation. Comptes Rendus de l’académie des Sciences Paris, 175: 411-413.

WOLFF CF (1759). Theoria Generationis. Halae. WOLPERT L (1998). Principles of development. Current biology. London/Oxford: Oxford University Press.

Received: January 31, 2006 Accepted: June 26, 2007