Book - Outline of Comparative Embryology 2-7

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Richards A Outline of Comparative Embryology. (1931)
1931 Richards: Part One General Embryology 1 Historical Development of Embryology | 2 The Germ-Cell Cycle | 3 Egg and Cleavage Types | 4 Holoblastic Types of Cleavage | 5 Meroblastic Types of Cleavage | 6 Types of Blastulae | 7 Endoderm Formation | 8 Mesoderm Formation | 9 Types of Invertebrate Larvae | 10 Formation of the Mammalian Embryo | 11 Egg and Embryonic Membranes | Part Two Embryological Problems 1 The Origin And Development Of Germ Cells | 2 Germ-Layer Theory | 3 The Recapitulation Theory | 4 Asexual Reproduction | 5 Parthenogenesis | 6 Paedogenesis And Neoteny | 7 Polyembryony | 8 The Determination Problem | 9 Ecological Control Of Invertebrate Larval Types

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This historic 1931 embryology textbook by Richards was designed as an introduction to the topic. Currently only the text has been made available online, figures will be added at a later date. My thanks to the Internet Archive for making the original scanned book available.
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Part Two Embryological Problems

Chapter VII Polyembryony

By polyembryony is meant among zoologists the production of multiple embryos from a single egg. (This definition does not apply to botanical nomenclature.) The number of individuals produced from a single egg ranges from hundreds as in the parasitic Hymenoptera down to two, although in the latter case the condition is usually known as twinning, and not all types of twins are properly thought of as polyembryonic.- Polyembryony occurs very widely distributed through the animal kingdom in groups which are totally unrelated to each other. Because of this wide distribution the literature to which reference can be made in this connection is really very extensive and the significance of some of the cases extends beyond the realm of embryology. Of the numerous investigators who have studied the question only a few need be mentioned here, but it may be noted that a reference to their work will serve as an introduction to the much broader literature which is not included in this discussion. Patterson has summarized the knowledge of polyembryony in a paper in Volume II of the Quarterly Review of Biology and it is suggested that the student consult this paper in beginning a more extensive study of these problems.

Patterson recognized three types of polyembryony: “ (1) experimental polyembryony, or the production of multiple embryos by artificial means; (2) accidental or sporadic polyembryony, or the occasional production of multiple embryos in the species which is typically monembryonic; (3) specific polyembryony, or the habitual production of multiple embryos in a given species.” The general student of comparative embryology is interested in the last type much more than in the first two, although cases of the first two throw much light upon many fundamental problems of organization of the egg and embryo and offer a very stimulating field for the experimental embryologist. Many types of eggs whose normal development includes no hint of polyembryony can be induced to produce double monsters and even complete embryos by experimental means.

Cases of occasional or sporadic polyembryony are recorded for coelenterates, cestodes, annelids, echinoderms, arthropods, and vertebrates. Perhaps because the latter group is so much more studied, there have been more cases reported for the vertebrates than for the others and, as Patterson points out, it is here that the occasional identical twins of the human species should be classified.

Specific polyembryony, to which our attention is drawn as the special problem of the comparative embryologist, is found in the following groups of animals: flatworms, bryozoa belonging to the Cyclostomata, earthworms, parasitic Hymenoptera, and the mammals, especially as illustrated by the armadillo. It is of more widespread distribution, however, than is indicated by these groups, but here are included the more important cases.

An inspection of this list is sufficient to show that polyembryony has no phylogenetic or taxonomic significance, for the groups are too widely separated and too diverse in structure to admit of an interpretation of this kind. Moreover, the structural differences both of adults and embryos in these various groups are so great that it is not possible to look for the causes of the phenomena in any simple embryological process common to all. Doubtless in the final analysis all these processes have a common underlying causal factor which is bound up with the innate protoplasmic organization of the egg substance.

As Patterson has pointed out, it is really not much more strange for an egg to give rise to two embryos than to one. The remarkable fact is that a new organism can be produced from an egg at all, and those qualities of the living substance which make possible its reproduction are basic to the one case no more than to the other. We must recognize that specific polyembryony is as much a characteristic of certain animals as is the formation of a coelome. At a certain stage in development there occurs a series of events which lead perhaps to two buds, or more, from each of which individuals grow. Very little thought is necessary to convince oneself that the directive forces behind this type of budding are not much more astonishing than those which cause, in the other case, the archenteron to bud out to form an enterocoele.


Of the explanations of polyembryony, probably the most prevalent is the theory of blastotomy, according to which the blastomeres of the cleaving egg become separated in the 2-, 4-, 8-cell stage or later, and lead entirely independent existences, each arriving at length at the stage of a completely formed embryo. This theory is the basis of the familiar explanation of the origin of identical twins in human beings, that the blastomeres in the 2-cell stage become separated and each gives rise to an embryo. This is a view with few observational data to support it, but it may be presumed to find considerable support in the experimentally induced development of isolated blastomeres. Numerous experimental devices are well known by which blastomeres can be separated. To mention only two of these devices as examples, it is possible more or less successfully to cut apart the blastomeres with a knife or delicate thread drawn about the egg, or to keep them apart by subjecting the eggs to calcium—free sea water in which the blastomeres do not cohere to one another. In some of these experiments the isolated blastomeres have lived and developed for a time; in others they have produced partial larvae (ascidians etc.); and in others they have developed into whole larvae (amphioxus, Cerebratulus). But in forms where direct evidence for blastotomy is possible none has been found which forces one to this view for cases of polyembryony. As will be seen presently, the facts in the armadillo make the explanation based on blastotomy untenable there, and render it highly doubtful elsewhere.

A second explanation takes the form of an assumption that polyovular follicles, that is, follicles containing several ova fused together, may provide the mechanism by which polyembryony is accomplished. It is only necessary to say that the evidence for this supposition is entirely insufficient and the theory may now be regarded as abandoned.

In the budding theory we have what is perhaps a closer correspondence with the observed facts, although it is more of a descriptive explanation than an attempt at developmental analysis. After cleavage there arise in the blastulae or gastrulae of lower animals or in the blastocyst of mammals certain areas or “growing points” which are essentially buds. If a single one is produced it becomes the apical or head end of a normal embryo and assumes dominance over the remaining parts. If two are formed they are the beginnings of twin embryos. Similarly secondary buds may be formed and four or more embryos result, depending upon the number of buds. The facts observed in the armadillo will be seen to harmonize well with this explanation.

This ‘theory of budding has been criticized rather severely, particularly by Assheton, on the grounds that budding cannot take place unless there is a stock from which the buds may arise and the presence of a stock Assheton claims was not demonstrated in the cases to which the theory was applied. Rather he interprets the described facts as cases of fission. The blastocyst of several mammals, for he was particularly considering the theory as applied to the armadillo, he described as undergoing direct fission into two embryonic rudiments. As a matter of fact the fission hypothesis does not differ from the idea of budding in relation to the facts as described, but is primarily a redefinition of them, for in both cases the embryonic vesicle is regarded as divided into several primordia, each of which is the beginning of a distinct formative area and consequently will give rise to a separate embryo. Whether the emphasis is placed upon the embryonic mass which is thought to bud or upon the separate areas which arise from it as would be the case in fission does not seem to involve any particular difference in the result nor to carry the explanation of the environmental processes much further toward a final analysis.

As an explanation of the process a physiological interpretation must be called on. Stockard has pointed out that if a developmental pause occurs at the critical moment in the formative stages of an embryo, a series of consequences ranging from simple malformation up to the production of double monsters or even two individuals may result. He applies this conception to the explanation of polyembryony, supposing that the reason for the loss of dominance of the first formative region of the embryo and the subsequent ascendency of the primary and secondary buds is connected in some way with the developmental pause. He points out that in the armadillo a period of physiological isolation intervenes at one of the critical moments, making possible the four resultant buds. Certain observed facts bear out this conclusion. In any case within the growing germ the isolation of formative areas occurs and from these formative areas the multiple embryos are produced.

An obvious connection exists between the phenomenon of polyembryony and that of metagenesis which involves at least the alternation of one sexually produced generation with an asexually produced one. It is usually the practice to limit alternation of generations to the lower forms of animal life and to say that it does not occur in higher forms, including the vertebrates. Stockard, however, speaking in terms of the budding hypothesis, has pointed out the fact that the embryonic mass of cells may be looked upon as one generation, namely the one produced from the fertilized egg, and that it is completed when it becomes a stock from which other buds, that is growing points, begin their development. This point of view has its application even to mammals whether only a single organism results or several as in the case of the armadillo. Here the blastocyst would be regarded as the sexually produced individual and the buds which give rise to the four young as those asexually produced from it.

Stockard says, “From a general biological standpoint the adult body of higher animals may be very correctly considered to be derived from a sexually produced embryonic axis, the stock which gives rise by an asexual method of budding to the various special organs. The vertebrate body is thus composed of a group of different zooids, the organs. There are seeing, hearing, excretory zooids, and so on, comparable to the zooids of a siphonophore colony.

“Alternation of generations is here considered a phenomenon, not limited as is generally taught to lower forms, but occurring throughout the animal kingdom.”

If the blastocyst is to be looked upon as a sexually produced individual from which arises asexually another or several others of a second generation, it is suggested that in this fact lies the explanation

fiG. 219. The development of the ovicell in Crista remosa. (From Patteron, after Harmer.)

A, external view. B, median longitudinal section of young ovicell. fol., follicle formed from polypide-bud; ov., ovary; ovi., oviccll.

of the difliculties that have met every attempt to homologize the early development of the mammals with that of the lower vertebrate classes. These differences have been discussed in Chapter X on the formation of the mammalian embryo; they constitute a problem of major importance to the comparative embryologist.


We may now devote ourselves to the consideration of the groups of organisms in which specific polyembryony has already been noted to occur, bearing in mind the different types of explanations that have been offered and attempting to discover how the facts conform to the suggested explanations.

flatworms. Among the flatworms are many cases which depart from the usual methods of reproduction. Of these some which do not fully fit into the classification have been described from time to time as polyembryony. There is one flatworm in which polyembryony undoubtedly occurs, however; it is the cestode Taema echmococcus, or Echmococcus coenurus as it is sometimes known. This small tapeworm produces eggs which develop into the usual hooked embryo, the onchosphere, and these in their turn produce a cyst which becomes the bladder worm or cysticercus. As is usual in the further development of a cysticcrcus, an invagination from the outside wall of the bladder into its cavity develops a new scolex and neck region which will later become inverted and form the new tapeworm. However, the matter is complicated in this case in that many scolices may be produced within a single bladder by the budding process, and even secondary bladders which produce multiple scolices in their turn are described. Thus the encysted worm may grow to an enormous size to endanger the life of the host and oftentimes bring about its death.

Bryozoa. Polyembryony occurs in the cyclostomatous bryozoa belonging to the Gymnolaemata of the Endoprocta. These animals live in colonies made up of zooecia

fiG 220- Section through which are in general tubular with densely

a follicle of Cnaza remosa show- - mg primary embryo (1) which calcareous walls. The circular orifices of the

§>ré>dt:)c1I(IiL(zisecorZ<I‘i‘ary erlgnbgnis zooecia give the group their name. At the time son, Xftei’ H$fi;e,_,r°m 3 e ' of the breeding season a curious specialized zooecium develops which is called the oecium

or ovicell. In this is developed the young embryo. Fertilization of a rather unusual sort having been successful, the oecium serves as a brood chamber about the developing embryos and larvae. The primary embryo undergoes a process of budding, and secondary and tertiary buds are EARTHWORM 359

produced all of which may develop into larvae, as many as 150 being on record from a single egg. They develop into ciliated larvae and each is capable of forming a new colony upon escape from the oecium. Earthworm. In the earthworm of the species Lumbricus trapczoides (Helodrilus caliginosus trapezoides) polyembryony results in the production of twins, a process which by Kleinenberg was regarded as universal for the species, but by Vejdovsky was held to be abnormal. It is due to a process of fission of the embryo. The cleavage of the egg

fiG. 221. Twinning in the earthworm, Lumbncus trapezoides. (From Patterson, A, B, C. after Kleinenberg.)

A, Section of young twin embryos. the right one being the more developed. B, Double embryo, in section. C, Double embryo about to break apart. D, Type of double monster formed when embryos fail to separate.

is much modified from the typical spiral form of the annelids and indeed is variable. A blastula is formed, the endoderm and mesoblast cells pass into the cavity and the entire mass begins to elongate. Across the equator of the elongating mass a transverse furrow now appears from one side. As it deepens, the embryo is divided into two hemispheres held together by a few ectodermal cells only. Each half is destined to form one of the twins. Differentiation, gastrulation, and the completion of the internal organization go ahead while the worms remain connected. But at length they separate after a series of rotations which breaks them apart, except in a certain proportion of cases which become double monsters of various degrees of union. Thus from the single egg twin earthworms are produced.

In two other genera of oligochaetes double monsters have been described, and one infers that the twinning phenomena, though rare, may occur in a number of families. These cases were Tubifea: tubifex, found by Welch, Tubzfex rivulorum by Penner, and Sparganophilus eiseni by Hague.

Parasitic Hymenoptera. In the parasitic wasps we find some of the most important and complex cases of polyembryony in the entire animal kingdom, and certainly here the results of the process are exemplified in the most striking manner. In a single brood hatching from one parasitized caterpillar hundreds of individuals may be seen to issue all developed from one egg. Our knowledge of polyembryony in insects dates from the work of Marchal in 1898 and of Silvestri in 1906. Although a number of important points still remain to be satisfactorily cleared up, a number of investigators have contributed to this subject since then. Among them are Martin (1914), Patterson (1915, 1917, 1921, 1927), Hill (1922, 1923), and Leiby and Hill (1923, 1924), as well as others. The important genera studied include Litomastix truncatellus, Ageniaspis fuscicollis, several species of Copidosoma, Paracopz'dosomopsis floridanus, and three species of Platygaster. The most recent study is that of Parker (1931) upon the braconid, M acrocentrus. Undoubtedly many other forms of insects show polyembryony, but the ones mentioned include the more important cases studied.

The manner by which a single egg produces multiple embryos in these forms is perhaps best understood by beginning as Patterson has done in the paper earlier referred to with a description of Platygaster hiemalis, a parasite of the Hessian fly, which usually produces two individuals from one egg. From this comparatively simple case we have a gradual increase in complexity which at the end of the series is only incompletely understood, but the simpler cases at least are suggestive of the methods by which the more complicated probably develop.

In this species the parasite lays from four to eight eggs either in the egg or young larva of the host. If fertilized, the egg nucleus divides twice, producing two polar body nuclei; these are not passed to the outside of the egg but remain in the cytoplasm where after a time they come together to form a large polar nucleus called the paranucleus. With the fusion of the male and female pronuclei the egg becomes differentiated into two separate regions. The cleavage nucleus and cytoplasm surrounding it become cut off from the remainder and constitute the embryonic region, for only from it comes the material which will go into the formation of the developing embryos. The rePARASITIC HYMENOPTERA 36 I

mainder of the egg containing the paranucleus and its own cytoplasm entirely surrounding the embryonic region functions to absorb and

fiG 222. Polyembryonie development of Platyoaster hwmalw (From Patterson, after Leiby and Hill )

A. Egg of four hours, showing sperm head and first maturation division B, Egg showing two polocytes. and both pronuclei C, Egg with pronuclei in comunction and polar nucleus D. Embryo, two polar nuclei and a. parasitic body showing four nuclei E, Enclosed within a cyst formed from tissue of the host are two embryonic bodies each surrounded by trophamnion F. A later stage in the development of two embryos G, A thirteen-day-old polygerm in section, showing several embryos.

elaborate the tissues of the host for the nourishment of the young embryos and it is therefore spoken of as the trophamnion. In this species, although it is not characteristic of all, the host tissues form a cyst wall around this developing body which then is spoken of as the parasitic body.

The paranucleus as development proceeds divides amitotically twice and the daughters distribute themselves in the trophamnion surrounding the embryonic region. The zygotic nucleus of the embryonic region divides first into two, then four, embryonic nuclei. Thereupon, the embryonic region becomes constricted and presently separated into two regions, each provided with two nuclei, and the trophamnion with its paranuclei also divides into two corresponding regions. The two halves of the parasitic body thus differentiated remain held together within a single cyst of host tissue, but develop independently. Four, eight, and sixteen nuclei arise from the division of each of these germs, as they are called, and these arrange themselves into the form of a typical spherical blastula, the cell walls being cut off around each nucleus. The remainder of the development is unimportant, for the process characteristic of these blastulae go forward through regular stages. There is produced from each a new individual which hatches in the next year.

In another species of this genus, P. vernalis, eight embryos are regularly developed. Here the process has a fundamental similarity to that just described, but the details of development differ. The parasitic body develops a number of embryonic nuclei surrounded by an appropriate amount of cytoplasm and a cell membrane. Each of these becomes a germ for the production of the later embryo and the entire mass is spoken of as a polygerm. It happens that the more complex cases were studied before the development of Platygaster was understood and some of the problems that were not clearly worked out would now probably be more easily followed through. It is known now that in Copidosoma gelechiae in the formation of the polygerm many primary germs are produced in a manner similar to the eight of Platygaster vernalis. These separate germs of the last polygerm stage all divide and give rise to two which develop into separate embryos.

Leiby and Hill believe that in Paracopidosomopsis, which is even more complicated, a secondary germ divides to form tertiary ones before the larval differentiation begins.

Armadillo. The final case of polyembryony to be discussed is perhaps the one in which the greatest interest lies. The Texas nine-banded armadillo Dasypus (Tatusia) novemcinctus normally produces four young from a single egg, the quadruplets being identical in respect to sex and to most of their morphological features. The work on the armadillo from the standpoint of polyembryony is of rather recent date, Fernandez concluding in 1909 that it occurs in a South American species Dasypus hybridus, and Newman and Patterson in the same year publishing the beginning of their important studies on the Texas species. Subsequent publications by these authors working independently have made available an account of the development of this form which is practically complete. The processes have been shown to be identical in respect to all important features in the two species which have been studied. The reader is referred to Chapter X for a discussion of the formation of the blastocyst in mammals.

In the armadillo as in other mammals, a monodermic blastocyst is formed which becomes differentiated into a trophoblastic portion and a formative portion, the inner cell mass. Then from the inner cell mass the endoderm is differentiated off, the remainder becoming ectoderm. Up to this point the blastocyst has remained free in the uterine cavity, but now attaches to the uterine mucosa. Attachment takes place directly over the embryonic ectoderm and the Trdiger forms at this point; then the inward growth of the spherical endodermal mass begins which brings about the so-called inversion of the germ layers. Within the ectodermal mass the formation of the ectodermic vesicle marks the beginning of the amniotic cavity. Above it an extraembryonic cavity is formed in the mesoderm. With progressive development the shift of the ectodermal cells takes place, resulting in the formation of the em~ bryonic shield with its thick ectoderm and the true amnion above it. In this stage the first sign of polyembryony makes its appearance, for from opposite sides of the ectodermal vesicle thus formed blunt projections extend laterally. Patterson called these primary buds. By this time that portion of the trophoblast which has not become involved in the attachment to the uterine wall disappears and the yolk sac endoderm is directly exposed in the uterine cavity. The two primary buds now divide each to form two secondary buds, making in all four buds which are rudiments of the four embryos subsequently to arise. Although they arise in this bilateral fashion and develop in the right and left halves of the uterus respectively, they later come to occupy positions that are about equally spaced from each other.

Meanwhile the vesicle grows, and the endoderm of the yolk sac, including those portions with which the embryonic ectoderm of each bud is in contact and which will form the gut endoderm of the embryo, faces the uterine cavity. The anterior ends of all the embryos point toward the apex of the common ectodermal vesicle, that is, toward the original amniotic cavity, and the entire vesicle may now be spoken of as the common amniotic vesicle. The entire structure now grows very rapidly, especially that portion of it which originated from the trophoblastic knob or the Trdger. This growth finally comes to occupy most of the space in the fundus of the uterus. The further history is especially concerned with the embryonic buds. Their posterior ends lengthen out with the growth of the vesicle and finally unite at a point opposite the original attachment and from their point of union the umbilicus later arises. The common amniotic vesicle by this growth procedure is left

fiG. 223. The development of the blastocyst of the armadillo. (After Patterson.) ec., ectoderm; en., endoderm; icm., inner cell mass; mes.. mesoderm; tr., trager; tro., trophoblast; u.. uterus.

as a small structure at the lower apex, in contact with each bud, while most of the remaining portion of the cyst mass with its developing embryos is derived from the growth of the Trdger. The buds meanwhile continue their growth, each forming a primitive streak, which is a substitution for the embryonic shield as described for other mammals. The further development of the embryos and the subsequent changes which lead to the production of the four foetuses need not be traced since it is simply a problem of organogenesis and of the steps normally following. The relations which are necessary for the understanding of

fiG. 224. A, The uterus and entire blastocyst of the armadillo showing paired origin 01' embryos. B, Half-grown foetuses spread out from uterus wall. (After Patterson.)

the polyembryonic condition all grow out of the budding processes of the embryonic vesicle and are completed with the establishment of the four embryos. There are many interesting problems connected with the later stages from the standpoint of later embryology and hereditary correlations of the characters of the four offspring as well as from other points of view, but the special problem of polyembryony is explained by these earlier stages.


Two other cases which resemble polyembryony in some respects should be mentioned. As a matter of fact there are some departures in each of them which do not justify their inclusion with this type of development.

Among the trematodes the egg of Fasciola hepatica, the sheep fluke, is fertilized in the body of the adult fluke, makes its way down the bile duct and out of the intestine of the sheep and hatches when it rains as a tiny ciliated larva, a miricidium. Entering the intermediate host, the snail, the miricidium becomes a sporocyst and within it parthenogenetic ova appear which develop into rediae, the next larval form. Within the body of the rediae the process may be repeated but at length cercariae are developed which escape from the snail and bring about the reinfestation of the adult host. This case has been called polyembryony by some, but by others it is regarded as paedogenesis, for here the new larvae are said to be produced not asexually but from parthenogenetic ova, a difierence in method which, if true, would seem to be fundamental. However, the reservation must be made that if the germ masses from which the new larvae arise are not ova, but are produced asexually (as F. G. Brooks believes, having found no maturation phenomena or other egg—like behavior), then we have to do here merely with asexual reproduction, and not even paedogenesis,

A second case which at first thought suggests polyembryony occurs in the tunicates. Among the Thalaceae as represented by the genus Salpa it has long been known that two forms of individuals are to be expected, one a solitary form and the other a colony which is usually found as a chain of individuals. The first of these develops asexually, although produced from a fertilized egg. The second is known to be budded ofi’ from a stolon which grows out from the asexual individual, and among the colonial forms some at least are sexual, producing the eggs and sperm which in their turn start the cycle over again. The genus Dolielum is rather more complicated than Salpa but it seems to illustrate the situation quite well and it is here that the type of development of particular interest to us may be said to occur.

In Doliolum an oozoid is developed from the fertilized egg and it reproduces asexually. By some it is regarded as a larval zooid, and certainly it has not yet reached its final character. If it is correct to regard the oozoid as a larval form, this case is very closely akin to polyembryony. From the ventral posterior part of the oozoid a proliferating stolon is formed as a protrusion of the ectoderm into which the mesoderm penetrates. From the stolon buds arise which break loose in a very immature condition and Worm their way upward and dorsally over the surface of the parent. Since the number given off from the ventral stolon is less than the number later found on the dorsal process, it is assumed that the buds must divide en route, and some evidence of this process has been found. Arrived at the dorsal side of the oozoid, the buds attach themselves to a posterior process of the test which arises as a middorsal projection (occasionally spoken of incorrectly as a dorsal stolon). These bodies attach themselves in three longitudinal rows to the outgrowth. The two lateral rows of buds are known as trophozooids or gasterozooids and their only function is to nourish the colony. The dorsal row of buds undergoes metamorphosis into several kinds of zooids. Some become phorozooids or nurse zooids, and according to some investigators they produce from a ventral stalk buds which are protogonozooids or primary sex buds. These latter either by budding or fission produce the sexual animals, the gonozooids or blastozooids, in which with further development sex organs are formed and the cycle is completed. Here an alternation of generations occurs in which there are three asexual generations and one sexual generation in one cycle. Whether it is to be regarded as polyembryony depends upon the interpretation of certain stages. It is very complicated in the extreme, and is at least close to true polyembryony.

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Pages where the terms "Historic" (textbooks, papers, people, recommendations) appear on this site, and sections within pages where this disclaimer appears, indicate that the content and scientific understanding are specific to the time of publication. This means that while some scientific descriptions are still accurate, the terminology and interpretation of the developmental mechanisms reflect the understanding at the time of original publication and those of the preceding periods, these terms, interpretations and recommendations may not reflect our current scientific understanding.     (More? Embryology History | Historic Embryology Papers)

1931 Richards: Part One General Embryology 1 Historical Development of Embryology | 2 The Germ-Cell Cycle | 3 Egg and Cleavage Types | 4 Holoblastic Types of Cleavage | 5 Meroblastic Types of Cleavage | 6 Types of Blastulae | 7 Endoderm Formation | 8 Mesoderm Formation | 9 Types of Invertebrate Larvae | 10 Formation of the Mammalian Embryo | 11 Egg and Embryonic Membranes | Part Two Embryological Problems 1 The Origin And Development Of Germ Cells | 2 Germ-Layer Theory | 3 The Recapitulation Theory | 4 Asexual Reproduction | 5 Parthenogenesis | 6 Paedogenesis And Neoteny | 7 Polyembryony | 8 The Determination Problem | 9 Ecological Control Of Invertebrate Larval Types

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