Book - Outline of Comparative Embryology 2-1

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I have decided to take early retirement in September 2020. During the many years online I have received wonderful feedback from many readers, researchers and students interested in human embryology. I especially thank my research collaborators and contributors to the site. The good news is Embryology will remain online and I will continue my association with UNSW Australia. I look forward to updating and including the many exciting new discoveries in Embryology!

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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Chapter I The Origin And Development Of Germ Cells

A problem of very considerable embryologieal interest is that of the origin and subsequent history of the germ cells. This is a matter of great theoretical interest because of its bearing on the Weismannian doctrine of the continuity of the germ plasm, and one which has been the cause of much controversy, at least as regards certain of its aspects. There are really three important phases of the problem; the place of origin of the germ cells, the path of their migration to the definitive gonads, and the subsequent history after reaching the germ glands.


A. VERTEBRATES

In Vertebrates the gonads arise from the genital ridges which develop on the dorsal side of the body cavity between the mesentery and the beginning kidney. This ridge is covered by the epithelium of the coelome or peritoneum, which is known as “germinal epithelium.” From the differentiation of this germinal epithelium in the four-day chick embryo Waldeyer (1870) first described the beginning of the sex cells or germ cells. Waldeyer’s view was accepted by the workers of his time as an adequate explanation of the origin of the germ cells and has been given as the only explanation in all of the text-books until quite recently. It is, moreover, supported by the observations of a school of investigators of the present day. It is, of course, true that with the formation of the gonad the germ cells are to be found in that organ and they appear to have arisen from its epithelial covering.

In 1880 Nussbaum, as a result of his observations on the development of the trout and the frog, reached the conclusion that the germ cells were of blastomeric origin, being segregated in the early stages of deVelopment in a region outside that which will see the formation of the embryonic body. They actively migrate or else are carried into the body proper and into the germ gland by the growth forces which operate to give the organism its body form. Since Nussbaum’s time the evidence has steadily accumulated that in many vertebrates, as in well-known cases among the invertebrates, the germ cells are early set apart from the somatic cells and develop independently from them. Their history may be traced backward from their appearance in the germ gland to very early stages and the path of their migration clearly made out in many cases. Many careful investigations (Hoffmann, 1892; Eigenmann, 1892; Beard, 1900; Woods, 1902; Allen, 1906, 1907, 1911; Dodds, 1910; Swift, 1914, 1915, 1916; Jordan, 1917; Okkelberg, 1921; Richards and Thompson, 1921; Swingle, 1926; Hahn, 1927) have failed to reveal any conditions not in accord with Nussbaum’s theory. It has been demonstrated that the primordial germ cells arise widely scattered in the


In. 125.2 Tr.ms\ erse section through .1 4 mm land of the bowhn Anna uzlza (After Allen )

Gut end . gut cudodorm roof end , roof endoderm, penph end, peripheral ondodcrm, Wolff cl . Wolfhan duct, s c . sex cells, lat mes , lateral mesoderm

endodeun or in the-splanchnic mesoblast in the extraembryonic area. From this point of origin they pass with progressive development to the region of the body axis. This movement has been accounted for in three ways; it is usually thought that the cells actively migrate in an amoeboid manner, passing up the mesentery and into the region of the gonad. Swift found that the primordial germ cells of the chick are carried in the blood stream, and in Fundulus it has been shown that the forces which are responsible for the formation of the embryo body carry the germ cells to their place. The movement of the cells of the germ ring by confluence and the other normal processes which lead to VERTEBRATES 273

the formation of the axial region of the body brings about the translocation of the germ cells as well as the somatic cells. Along the path of migration there is no difficulty in recognizing the germ cells for they have clearly marked characteristics. On these points there is rather general agreement.

The subsequent history of these cells appears to be an uncertain prob1em———one much involved at the present time in controversy. Some investigators claim that some of these. cells never find their way into the gonad, but undergo degeneration in various parts of the body. Some (firket, von Berenberg—Gossler, Ilargitt, and others) claim also that the primordial cells which may have arrived in the gonad by


flu. 183. 'l‘rans\'(-rse section through the hind gut of l).l—mm. larva of Amid crzlva. (After Allen.)

coel., coelomie cavity; gut end., gut endodorm; mes., mesentery; s.e., sex cells; Wolfi’.d.. Wolffian duct.

migration in the manner indicated _do not actually give rise to the definitive germ cells, for they find degeneration of these cells along with proliferation of a second group of germ cells derived directly from the genital epithelium. These latter, or even in certain cases a third proliferation, are the source of the definitive germ cells. The primordial germ cells in these cases may pass far through their cycle before degenerating, as shown by Kingery for the mouse and Swinglc for the tadpole, in both of which the maturation cycle is practically completed before the degeneration occurs.


In a recent account Swingle has reviewed the progress of the investigations on the Anura, He has shown from embryological studies that there are in various parts of the United States “local races which differ markedly in regard to the time of occurrence and character of the developmental processes involved in the formation of the definitive testes.” He is convinced that the sex cells appear first in the endoderm, and give origin to those cells which produce ripe sex products. Every stage from the origin of the primordial sex cells to the fully developed spermatozoa has now been traced in the bull frog, and no doubt now remains as to the continuity of the germ line in this form. On the other hand Hargitt and others have claimed that the germ cells arise in urocleles from differentiated mesoderm, the peritoneum. Swingle sug— gests that the discrepancy is due to an origin and segregation earlier than their appearance in the peritoneum, and that after their segregation they wait in the layer where they are later seen for an appropriate time for development. This is in agreement with his finding that local races differ in the time of development of the sex cells. In the urodeles they are incorporated in the mesoderm in an early stage whereas in the Anura they are in the endoderm, but it is not necessary to assume their origin in either layer because of the fact that later they appear in the one or the other.


It is to be noted that none of the writers who find degeneration have satisfactorily proven that all the primordial germ cells degenerate or that none of them are left to produce definitive germ cells. It is also noteworthy that recently certain evidence has been found which indicates that in an ordinary course of events the definitive germ cells are derived from the primordial cells. However, owing to the power of regulation which animals possess under the stress of unusual and harmful circumstances which bring about the degeneration of the primordial cells, secondary sex cells may be proliferated from the germinal epithelium and produce the definite sperms or ova. At the present time the safest interpretation seems to be that as a rule the primordial germ cells thus early segregated in the vertebrates give rise to the definitive germ cells, but that in exceptional cases, where the regular events are interrupted and the primordial cells so disturbed as to cause their degeneration, the organism responds by proliferating from the germinal epithelium new cells which take their place. It must never be forgotten that the chief characteristic of protoplasm, especially embryonic protoplasm, is its plasticity, its power of regulation and of adaptation to new conditions. “Life is a constant adjustment of internal and external conditions.”


Although the matter is still involved in controversy, some important theoretical conclusions have already resulted from the investigations. first of all, it is an obvious fact of major importance that heredity is a matter of germ cells, and that the germ cells contain organized material through which the continuity from generation to generation is carried on. The germ cell mechanism is the mechanism of inheritance. Not only is there continuity of germ cells but of the organized system (germ plasm) which they convey. Secondly, the distinction between germ cells and somatic cells is often more or less artificial and has in many cases been emphasized unduly. Weismann himself spoke with less certainty about the continuity of germ plasm than many of his followers have done, although he without doubt was expressing a fundamental truth. Yet there still remained a theoretical question which his formulation of his doctrine did not fully state, namely, the possibility of the soma contributing to or becoming part of the germ plasm. Various views on this point have already been given.


It is the author’s opinion that with the possible exception of the mammals no group has as yet presented evidence of positive character that contradicts the view that germ cells as well as germinal material are distinct from the somatic from a very early period. In the mammals the situation is 11ot yet clear. It is worthy of note that the cases in which uncertainty or confusion exists are exactly those which have proven least satisfactory for cytological and accurate embryological studies. When favorable material is available there is general conformity to the expectations of those who would emphasize the importance of the germ plasm. These include cases of determinative cleavage where the germ—cell cycle has been followed in case after case, and also those which have yielded clear figures for chromosome studies.

If in addition the recent view be accepted that the chromatin of all cells represents the germ plasm and the cytoplasm the soma, as is now held by numbers of students, the history of the germ cells becomes a study of the specialization of chromatin and nuclei and that of the soma one of cytoplasmic specialization. The general problem of the germ-cell cycle is thus given its proper place as one of embryology, where it is of great interest, and it is no longer one of theoretical significance to geneticists.

B. THE INVERTEBRATES

In the invertebrates there are many forms in which no suchMuncertainty prevails as in the case of the vertebrates. There are two chief methods by which germ path is recognizable: 1.e., chromatin diminution and germ-cell determinants. There are of course many cases in which nothing is known of the development of the germ cells, but there are in several groups of the invertebrates cases in which the germ path (Keimbahn) has been-worked out in practical completeness. In the coelenterates primordial germ cells were found by Weismann, Kleinenberg, and others in a wandering condition early in the development of hydroids. The details of Weismann’s observations have been disputed by Hargitt, but his main contention is borne out that the germ cells are recognizable long before the sexual individuals appear. The case

fiG. 184. The dexelopment of the germ cells of Sagilta. (After Elpatiewsky.)

A, fertilized egg showimz the “germ cell determinant,” B, 2-cell stage, germ cell determinant in one blastomere only; (‘, H—cell stage; D, szastrula with two primordial germ cells. '

t 2., germ cell determinant; p.b.. polar bodies; p.g.c.. primm'diul germ cells.

which has been of greatest interest from the standpoint of the history of the germ cells is that of Ascaris megalocephala as brilliantly worked out in great detail by Boveri. In Ascaris (see Chapter IV, Section 3) the first cleavage division results in two blastomeres, one of which is purely a somatic cell while the other is a stem cell. The history of each 'rm«: INVERTEBRATES 277

fiG 185 Early stages In the development of Cyclops fuscus (After Amma)

A, first cleavage nucleus, 13, (laughter nuelex reconstructed with the ectosomes or germ cell determrnants 1n one blastomere only, 0, second elenvage dnmon wrth the germ cell determmants agmn l1m1ted to one blmtomere, D, l()-cell stage All the blustomeres m the

restmg condltmn except the stem cell, E, gastrula In whlch two pnmordral germ cells are to be dlstmgulshed from the endoderm cells. 278 THE ORIGIN AND DEVELOPMENT OF GERM CELLS

of these cells has been fully described by Boveri. They are distinguishable by means of a process known as chromatin diminution, part of the chromatin (the thick ends of the chromosomes) of the somatic cell being cast from the nucleus into the cytoplasm, because of which process the


fiG. 186 A, longitudinal section through an oocyte of M iastor americami. (After Hegncr )

n c , nurse chamber, g v . germinal vesicle, p Pl , pole plusm, f ep , follicular epithelium B, later stage of same in which somatic cells are beginning to form and blastoderm and the two pl'll‘flB.!’y germ cells containing the pole plasm are already cut of?


somatic nuclei in subsequent cell generations are both lighter in color and smaller than the germ cells. All the chromatin of the stem cell is retained in this second division, but of the daughter blastomeres produced from it, one undergoes the diminution process while the other divides directly in the regular manner and becomes again a stem cell. Of the products of its division the same relation holds, for the process is repeated four times. Thus, at the end of the fourth cleavage there are fifteen somatic cells (which will give rise only to somatic cells ordinarily) and one cell which is the primoridal germ cell and the ancestor of all the germ cells. At the next division this divides again

Fm. 187. Continuation of fig. 186. A, similar stage of eight germ cells, blastoderm complete; B, sagittal section through embryo with eight germ cells near the end of the tail fold.

and the two cells thus produced give rise later on in the interior of the embryo to all its sex cells.

In Sagitta, Elpatiewsky recognized in the fertilized egg a structure, called a “germ-cell determinant,” which does not divide with the egg. (fig. 184.) The blastomere containing it becomes a stem cell. In the fifth division the stem cell produces a somatic cell and a primordial germ cell. 280 THE ORIGIN AND DEVELOPMENT OF GERM CELLS

In an early gastrula four cells have resulted from two divisions of the primordial cell. The subsequent divisions of the four are responsible for the formation of both male and female germ cells (for the animal is hermaphroditic). It is said that the two anterior give rise to the ovary and the two posterior to the testes.

Among the arthropods there are many cases of the early segregation of germ cells, and their recognition is possible either through cytoplasinic inclusions (germ-cell determinants) or by distinctive characteristics, particularly chromatin diminution. In the Entomostraea are two outstanding cases, M oina (Grobben) and Cyclops (Haecker, Amma). Here the stem cells are distinguished from the first cleavage onwards. In the unsegmented egg of (71/claps (fig. 185) certain granules (ectosomes) are to be found accumulating at one pole of the spindle as the cleavage mitosis advances. These granules are thus segregated into one of the blastomeres which becomes the stem cell. This process continues through the fourth division and results in the segregation of the primordial germ cells.

In the insects a germ path is recognizable in several orders. In the Diptera (Mzestor, Kahle, 1908, Hegner, 1914, Chironomus, Hasper, 1911, and in other cases) the germ cells appear at the

Fm. 188.

Longitudinal section through ovarian egg of ( hpixlosoma. almost ready to be laid. (After Hegner.)

The germ cell determinant (k) lies near the posterior end of the egg.


posterior end of the egg as pole cells and can be traced into the larval gonads. Here polar granules, germ-cell determinants, or pole plasm, as the mass is variously called, are present in the uncleaved egg and enter the pole cells. In addition to this in M iastor (figs. 186, 187) the pole cells are provided with two nuelei from the third cleavage which have not undergone chromatin diminution as distinguished from the other six. In the Hymenoptera. (fig. 188) an “oosome” is present in the unscgmented egg which in the 4—eell stage becomes associated at the posterior end with the forming primordial germ cells. finally, in the chrysomelid beetles (fig. 189), Hegner found a “polar disc” composed of germ-cell determinants which participate in the formation of the primordial germ cells. This relationship was made more certain by an experiment which he performed. With a hot needle he killed the posterior end of the egg, l)ut allowed the remainder of the egg to develop. A blastoderm was formed which was normal in every particular (up to a certain stage) except that it was entirely lacking in germ cells.


fiG. 189. Longitudinal section through the egg of Leptinotarsa drmlvnlirieata. (After Hegner.) The'posterior end was killed with :1. hot needle just after the egg was laid. The egg was then allowed to develop for 24 hours. bl., blastoderm; g.c.d., germ cell determinants; k.. portion of the egg killed.



These cases prove conclusively that among many invertebrates we may regard the early segregation of the germ cells as satisfactorily established. In addition to these there are many cases of cell lineage in which there is no question but that the germ cells arise from blastomeres segregated at a quite early stage. It is to be noted also that these cases are among the best known of all animals, the material being abundant and most favorable for the study of entire life cycles.

Historic Disclaimer - information about historic embryology pages 
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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


Cite this page: Hill, M.A. (2020, September 27) Embryology Book - Outline of Comparative Embryology 2-1. Retrieved from https://embryology.med.unsw.edu.au/embryology/index.php/Book_-_Outline_of_Comparative_Embryology_2-1

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