Book - Outline of Comparative Embryology 1-6

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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 One General Embryology

Chapter VI Types of Blastulae

It is customary to say that cleavage continues from the fertilization of the cell to the formation of the blastula. We usually think of a blastula as a hollow spherical embryo consisting of a single layer of cells, and indeed the most typical blastulae are so constructed. Actually, however, there are many different forms of blastulae, of which the hollow, onelayered type is but a single one, and these different types of blastulae are now to be .considered.

The different forms of blastulae occur as the end stage of many varied cleavage processes, as has been seen in the foregoing sections. The cleavage types are largely traceable to the amount and distribution of yolk present in the egg, a factor which differs widely in the several groups of animals. For the same reasons it is to be expected that the different cleavage types will lead to difierent kinds of blastulae, and this expectation is borne out. There are seven types of blastulae as follows: coeloblastula, stereoblastula, morula, placula, amphiblastula, superficial blastula, and discoblastula.


The simplest type of blastula is that known as a eoeloblastula, the kind having a cavity at the center. Coeloblastulae may be further subdivided, depending upon the size of the segmentation cavity and upon the relative sizes of the cells. They include both equal and unequal blastulae, distinguished as to whether the cells are of uniform size throughout the embryo, and each of these is again of two distinct subtypes, depending upon the size of the blastocoele cavity. If the altitude of the cells is low so that they resemble a sort of cuboidal epithelium, then necessarily the cavity is very large, and we have the simplest type, the equal eoel0blastula—the one with which we are most familiar, since it occurs in amphioxus. If, on the other hand, the altitudes of the cells are greater than their breadth, the cavity is correspondingly reduced; there is still a blastocoele but it is very tiny and is surrounded by a very thick cell wall (consisting nevertheless of only a single layer of cells). This occurs in Sagitta, a member of a group closely related to nematodes.

Usually in the equal coeloblastulae the cells are about the same size and in about the same condition. Very evidently the equal coeloblastula is a type in which there is very little differentiation among the cells of the embryo as the cells are in approximately the same condition and of about the same size. Those at the vegetative pole usually show a slight size difference because of their content of yolk, but these cells do not have much yolk and the distinction is not very great.

Equal coeloblastulae occur in the following groups but do not include all members of these divisions. They are found in coelenterates, in nematodes, Sagztta, echinoderms, Phoroms, brachiopods, Balanoglossus, and amphioxus. It will be seen that these forms have equal cleavage which is mostly radial or bilateral cleavage. (The spiral type, however, does not entirely lack the coeloblastula, and there are a few annelids

fiG 57 Blastula of amphioxus. Fm 58 (‘oc[0b1astu1,; of 30,. (After C0I‘f0flWm0) mtta (Redrawn from Korsehelt and Heider, after Hertwig )

which rather closely approach the coeloblastula type.) Gastrula formation in types having coeloblastulae is usually by invagination; however, the method to be discussed presently, polar ingrowth, sometimes applies to this type.

The unequal type of coeloblastula is derived from the equal, with increased difference in size of the blastomeres at the animal and vegetal poles. Those at the animal pole are shorter than those at the vegetal pole. The blastocoele is not often very large and, owing to the larger size of the vegetative cells, it is pushed into the animal hemisphere. Eggs of this kind are characterized by a more sharp differentiation between yolk cells which are to form endoderm and animal cells which are to form ectoderm. The eggs are mostly spherical, sometimes a little elongated, and because of the difference in yolk content variations appear, just as in the equal type of coeloblastulae; among them are variations in the thickness of the wall, and in the size of the cleavage cavity.

The unequal coeloblastula is widespread in eggs which cleave spirally.

Therefore we should expect to find it in the polyclads, annelids, molluscs, Bryozoa (fig. 59) and among the holoblastic eggs of vertebrates (fig. 26). The blastulae of vertebrates can readily be distinguished from the other types, for in them we always find a great many cells in the wall. In the vertebrates, epithelium is differentiated into many layers of cells. It is as if this differentiation that is characteristic of adult vertebrates makes itself felt even in the cleavage stages, so that the wall of the blastula consists of many cells, and each germ layer, ectoderm and endoderm, is several cells thick. This characteristic is clearly seen in the amphibian type, the frog and salamander, with such modifications as the difference in yolk would involve. Unequal coeloblastulae are derived from unequal total cleavage. Gastrulation is commonly through invagination, although polar ingrowth and epiboly also are found in these eggs.

fiG 59 Unequal eoeloblastula fiG 60 Equal stereoblastula of flustrella hzsmda (Redrawn of Lucernaruz (Redrawn from from Korschelt and Herder, after Kcrsehelt and Heider, after Pace) Bergh)


The stereoblastula (the term means a solid blastula) is obviously derived from the coeloblastula, and may be equal or unequal, depending upon whether it is produced from the equal or unequal type of c0eloblastula. If the cleavage cavity in the type of equal coeloblastula with small blastoeoele continues to be reduced until it vanishes, we have the equal type of stereoblastula such as is found in Lucernarza. With increase in volume of cells and decrease in size of blastocoele we get a stereoblastula.

More common than this, however, is the unequal type of stereoblastula such as we find in Crepzdula. The unequal stereoblastula is derived through unequal cleavage and may occur either in the spiral types or the more extreme of the bilateral types. Gastrulation regularly occurs in this type by epiboly. The spiral types lead easily to this type of blastula in which by continued growth the ectoderm cells will surround the endoderm cells and so form a gastrula by epiboly.


A second type of blastula in which there is no blastocoele is the morula. It was formerly customary to include this stage in almost all the types; that is, it was said that the morula followed the 16-cell stage, and in the strictly radial type there is some reason for that description, but we do not speak now of a morula stage. However, in some types there are formed blastulae which can best be described as morulae or solid blastulae. In Clava we have an example of the solid blastula which illustrates the difference from the stereoblastula type, for inside the morula there are cells, whereas in the stereoblastula there were no cells which did not

fiG. 61. Unequal storeoblustulu of Crepidula. (After Conklin.)

have a boundary at the surface of the egg. Superficially the two are quite alike, but actually there is an important distinguishing feature. The morula is produced by a characteristic shifting of the spindle position during cleavage. In all the other cases that we have dealt with the spindle may be said to occupy a paratangential position; the result is that the new cleavage plane divides the cell in such a way that part of it touches the surface and part the center of the egg. However, the morula comes about by a shifting of the spindle to a radial rather than a paratangential position, and the result is that some of the daughter cells touch the surface and others are only in the interior of the egg. So the morula, although superficially similar to the stereoblastula, is really a distinct and different type. Morulae occur chiefly in coelenterates.


The placula presents another kind of modification which has been produced from the equal coeloblastula. Let us suppose that, instead of the cavity vanishing, the egg is compressed from the animal to the vegetative pole. The result would be a flattened blastula consisting of two plates of cells. This is the placula and is found especially well developed in the ascidians and to a less extent in the nematodes. The edge of the placula corresponds to the equator of the coeloblastula, the upper layer

of cells to the animal hemisphere, and the lower layer to the vegetative hemisphere.

Tie. 62 Morula of (‘lava 9qua- fiG 63 Placula of Cynthia pamta (After mam (Redmwn from Korschelt and Conklin) Heider, after Harm )

Gastrulation takes place by two different methods, epiboly and the sinking in of the flattened plates; that is, it is accomplished by a combination of epiboly and invagination.


The amphiblastula of the sponge is of somewhat different type. This structure was first described by Haeckel for the free-swimming larval stage of calcareous sponges, and hence does not strictly correspond to other blastulae which we have described. This larval form, however, is usually spoken of as an amphiblastula. The cells at the animal half are very sharply distinguished by their small size and ciliated covering from those in the vegetative part of the egg. This type is not particularly important although it is necessary to include it.


The superficial blastula, or periblastula, is characteristic of those forms having superficial cleavage, an example of which is the crab, Dromza (figs. 36f and 37d). The result of superficial cleavage is the formation of a layer of cells around the inner yolk mass. There is no blastocoele and it is usual to say that the blastocoele is occupied by the yolk mass. But the case is not quite so simple. Actually it will be recalled that the superficial cleavage resulted in the formation of blastomeres which at first were continuous with the yolk at their inner end. These blastomeres presently are completely out off, a protoplasmic blastomere being separated from the yolk-laden inner mass. Their inner ends, or the outer part of the yolk mass, therefore, is not homologous to the cavity. It is only that very small part of the mass which was never segmented at all which is homologous to the cavity.

Reference has already been made to the role of the yolk cells, the vitellophages within the yolk mass Some of the nuclei during cleavage remain behind and are presently to be described as yolk cells for they

fiG 64. Amphiblastula of Sycandra mphanuv (Redrawn from Korschelt and Helder. after Schulze )

digest the yolk. In this process the yolk becomes divided into areas around these nuclei, a process of division which is often spoken of as the yolk cleavage. But by some these nuclei are regarded as endoderm. If this latter VIOW is adopted, there would be no such thing as a superficial blastula because the embryo would consist mostly of two cell layers, the outer being ectoderm, and these internal yolk cells endoderm. This designation of these cells as endoderm has not, however, come into general use. Rather we say that the superficial is a true blastula.


finally we have the discoblastula, the end stage of the eggs having discoidal cleavage. It is derived from the unequal coeloblastula (fig. 52) upon the assumption that the yolk becomes so abundant and so dense as to hinder quite completely the separation of the cells in the vegetative half of the egg. Recalling that discoidal cleavage occurs both in the vertebrates and invertebrates we should say at once that discoblastulae are of two sorts, the vertebrate type (fig. 55) and the invertebrate (fig. 44); the latter consists of the single layer of cells which is found in all invertebrates having diseoidal cleavage in contrast to the former which is of many cells. Usually in all types having discoidal cleavage there are some cells, perhaps around the edge of the disc or perhaps elsewhere, which are continuous with the yolk. These cells, however, usually do not take an active part in the formation of the body of the embryo. In the vertebrates these cells are designated as the periblast or the yolk sac endoderm. In the cephalopods they are the blastocones. Gastrulation is usually by the limited type of invagination, invagination taking place in one segment of the disc only.

These are‘ the different types of blastulae. They are obviously related by the differing types of cleavages and they lead us to the different types of gastrulation which is the next step in the development of the animal’s body.

Bibliographic Note

Among the more important accounts of the subjects contained in this chapter are the following: Korschelt and Heider, Minehin, Maas, Hertwig, Wilson, Conklin, Schultze, Brauer, Whitman and Eyclcshymer, O. Herting, Ziegler, Brooks and Bittenhaus. These works are cited in full in the bibliography on page 406.

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. (2021, April 16) Embryology Book - Outline of Comparative Embryology 1-6. Retrieved from

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