Book - Outline of Comparative Embryology 2-2

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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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Chapter II Germ-Layer Theory

It will be recalled that the old issue between preformation and epigenesis finally resulted in the overthrow of the cruder type of preforn1ation which held that development was simply an unfolding of parts already existing in egg or sperm. As better understanding of the facts of development was obtained it became clear that, although preformation did not occur, there were definite signs of organization within the egg. The developing organs could be traced farther and farther back until it was evident that in general they came from what might be called three plates or layers of tissue lying one over the other. These layers give rise by various processes of folding, outpocketing, unequal growth, etc., to the fundaments of the future organs. It was further obvious that there is a great similarity in regard to this method of origins from these layers in different kinds of embryos. These facts were known in whole or in part to many of the early embryologists, including von Baer (1828), and others, even back to the time of Wolff (1768). ()f course the proper interpretation of cells and cell structure in the animal kingdom had not as yet been made and it had proven impossible to explain the origin of the layers in terms of any other lower units of organization. It is not surprising, therefore, that these early embryologists should have looked upon the formation and subsequent development of the germ layers as the most critical processes of embryonic life.


The germ-layer theory, which is distinctly an attack on the problem of the organization of the embryo, has been one of the most fruitful of all biological speculations. After the true nature of cells and their methods of reproduction and growth came to be understood, the theory acquired new significance and, in the hands of Oscar and Richard Hertwig, had attained the status of a full-fledged theory of development by the early eighties. Through a great variety of forms, blastulae and gastrulae were identified, their methods of origin understood, and the remarkably uniform character of all the processes involved in germlayer formation and in the laying down of the fundaments of the organs was made out. Exceptions occur; indeed, some of them are of great importance, but considered as a whole there is striking uniformity in the relationships of the layers to each other and to the subsequently developing organs.


The essential point of the theory is that the three layers are homologous throughout the animal kingdom above the Porifera. In the coelenterates, as in the gastrulae, there are present only ectoderm and endoderm. Since the layers are homologous, the organs which arise from them are also homologous wherever they are found.

In all metazoa there normally develops following the blastula, a stage called the gastrula, of which the two component layers are known as primary germ layers, the outer being the primary ectoderm (epiblast, ectoblast), and the inner the primary endoderm (hypoblast or endoblast). The primary ectoderm in etenophores, turbellarians, rotifers, annelids, and molluses contributes with the primary endoderm to the formation of the middle germ layer, the mesoderm (mesoblast) giving rise to the distinction of ectomesodcrm and cndomesoderm respectively. From the outer layer also come: (a) the covering tissues with all their modifications and appendages including hair, horns, nails, scales, skin, glands, (b) the nervous system and sensory epithelium, and (c) in many cases the stomodaeum and proctodacum (extreme anterior and posterior regions of the alimentary canal).


The primary endoderm, in addition to cndomesoderm, produces the lining of the midgut and of all the organs which are derived from it, as pancreas, liver, etc. There is also exceptionally endoderm participation, for example, in the formation of a part of the nervous system in some coclentrates, but in some other cases, as in formation of blood cells and endothelium of blood vessels, the evidence is not yet clear as to whether it takes part.


The mesoderm may come, as already indicated, from the two primary germ layers, but except as mentioned above, only the endoderm seems to be involved in its production. There are several methods of mesoderm formation. Isolated endoderm cells may push into a space between the two layers at the same time with the secretion of a jelly-like substance, thus giving rise to a filling-in layer known as mesenchyme from which certain organs or parts of organs take their origin. In a second method the typical epithelial character of the primary germ layers is preserved and by outfoldings from the walls of the archenteron (primitive gut), which becomes entirely disconnected from it, lateral coelomic pouches are formed. This type of mesoderm is known as mesothelium. There are also other processes by which coelomic walls may in special cases be produced. The inner or splanchnic wall of the coelomic pouch unites with the endoderm to form the splanchnopleure, and the outer or somatic layer forms with the ectoderm the somatopleure. In some animals the middle germ layer is entirely mesenchymous; in others, entirely mesothelial; and in many, both types are present. Either one may develop first; in the echinoderms the mesenchyme arises before the mesothelium and in the vertebrates after it. The mesoderm gives rise to muscles, blood, skeletal and connective tissues, the excretory organs (at least in part) and usually to the sexual organs, but not the sex cells.


The surprising uniformity with which these relationships are held throughout the higher groups of animals has been responsible for the great importance of the germ-layer theory as outlined by the Hertwigs. Probably no other theory or working hypothesis, with the exception of Darwin’s own contributions, has been so fruitful as a stimulus for constructive zoological work. The germ-layer theory has to its credit much of the most important embryological accomplishment. As a working hypothesis it is of the greatest importance.


As an analysis of the problem of organization of the embryo, less can be said for the germ-layer theory. Researches of later years with careful technique have shown that the exceptions which formerly were overlooked are in many cases difficult to harmonize with the theory and, indeed, often operate decidedly to limit its usefulness.


Among the objections that have been cited are the following: inverse relationships of the layers found in the sponges; the fact that the outer layers of the (-estodes are in part or in toto thrown off leaving the covering tissues doubtfully related to the eetoderm; the varying derivation (depending upon the viewpoint of the observer) of the notochor(l and mesenchymous connective cells in different chordate groups; the composite nature of some organs, for example the nephridia, from more than one germ layer; the identification of the mesoderm as the 4d cell, not a layer at all, in the individuals with spiral cleavage; and especially the facts of budding, differentiation, and regeneration. Some of these objections have proven difficult to answer although this fact should not make us underestimate the value of this conception as a whole. However, it is now known that there are many evidences of organization appearing long before germ layers are formed. The promorphology of the ovum in the eggs with determinative cleavage and the possibility of tracing out the cell lineage during cleavage and organ formation in embryos of this class distinctly minimize the importance of the germ layers. Back of this the organization of the chromosomes and their part in shaping development as is now known from studies of cytology and genetics again detract from the value of the theory.


We have now come to view the germ layers as representing a stage in development just as we regard the blastula or gastrula as progressive steps. It is an essential stage in most cases through which the course of development must pass, but in special cases adaptive modifications have arisen. Homologies exist between blastomeres—sometimes, as in the 4d cell and the “cross” of the annelids and molluscs, of very striking character; they also exist between germ layers, between organs and between organ systems. All are important, perhaps equally so. But the germ layers are probably best regarded as temporary embryonic organs which play their part and give rise to subsequent stages of development rather than as units of organization which determine the future course of development.



Historic Disclaimer - information about historic embryology pages 
Mark Hill.jpg
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, March 28) Embryology Book - Outline of Comparative Embryology 2-2. Retrieved from https://embryology.med.unsw.edu.au/embryology/index.php/Book_-_Outline_of_Comparative_Embryology_2-2

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