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Embryology - 20 Apr 2024 Expand to Translate
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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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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)

Part One General Embryology

Chapter III Egg and Cleavage Types

Of the four stages of embryonic development, cleavage, germ-layer formation, period of development of organs, and of histological differentiation, the period of cleavage is the most constantly recognizable and the most distinct. It consists of a succession of cell divisions taking place in a very regular manner and having for their purpose the distribution of the egg protoplasm into a great many single cells. Every organism above the protozoa consists of many cells, and every one starts as a single-celled egg. The first problem of development for the fertilized egg, therefore, is that of the regular distribution of its substance in such a manner that the organs of the later stages may normally arise from the appropriate material. Since divisions follow one another with considerable rapidity during cleavage the daughter cells never have time to grow to the size of the mother cell. The cleavage cells or blastomeres normally divide into two cells each, thus setting up a normal rhythm of 2, 4, 8, 16, 32 cells, etc. Even in the simplest cases, however, this rhythm cannot long persist, for with the beginning of differentiation processes certain cells lag behind in the divisions and irregularities thus come in.

Cleavage usually results in spherical blastomeres which are in contact only at the point where the cytoplasmic division last cut through. ()wing to the tension and other physical factors, this condition at once passes and the blastomeres come to lie closely pressed together with only a furrow remaining to show where the separation really is. The furrow naturally falls perpendicular to the spindle axis of the last division. Often the plane of the first furrow corresponds to the median plane of the future embryo.

This last fact, along with others which indicate that there are, at least on many eggs, structural features that link up the uncleaved egg with the future organs developing from its various special regions, raises the question of the promorphology of the ovum. This matter is correlated with the preformation discussion to which reference is made elsewhere. It is often possible to trace the origin of organs or cell groups to very early blastomeres or even to the single-cell stage, a study which is known as cell lineage, and there are often in the egg marks of structural differences by which regional differentiation is indicated. The presence, for example, of yolk at the vegetative pole of the egg distinguishes the ectodermic and the endodermic portions at once. In ascidian eggs there are differences in the consistency of the protoplasm and in the pigments present, so that with the entrance of the sperm there can be distinguished four regions, namely, a clear protoplasmic cap at the animal pole, a darker area at the vegetative, and in between two crescents, one yellow and one light gray. There are thus localized certain organ-forming substances in four different regions, and these will give rise to very different parts of the embryo. For these reasons we say that there exists in the eggs of many species a predelineation of the structures which are to be developed. This is of course contrary to the assertion sometimes made that the blastornere is similar to the entire egg, is a small picture of the whole. This statement is true only in the sense that the egg commonly shows little evidence of histological differentiation, and the same is at least superficially true of the early blastomercs. That the resemblance is only superficial, however, is also borne out by the nuclei of certain forms. In Ascaris one of the two nuclei of the 2—cell stage undergoes a process known as chromatin diminution, becomes smaller, and stains more lightly than the other; this is indicative of the separation of the germ line from the soma.

Fro. 9. The progress of the first cleavages of the snail, Planorbis, showing the manner in which the furrows out through forming two spherical blastomcres. which then draw together, substituting :1. plane of contact for the point of contact.

As cleavage progresses the tension which keeps the entire protoplasmic mass nearly spherical imparts to the individual cells a tendency to approach each other and press together. Largely owing to this tendency the blastomeres arrange themselves at the periphery of the mass and leave a cavity at the center, known variously as a cleavage cavity, segmentation cavity, blastocoele, or primary body cavity. It is filled with fluid or with a gelatinous secretion. In eggs containing large amounts of yolk, however, the cavity is crowded out of the center and often greatly restricted by this deutoplasmic accumulation. The cellular surface of the embryo at this stage is known as a blastoderm, whether it involves the entire surface or is limited to a small area of it.

Cleavage may be said to begin with the formation of the first segmentation spindle. This is a more accurate statement than that it follows fertilization, for in the case of eggs which develop parthenogenetically there is of course no fertilization. It is more difficult to say when it closes, for any limit which we may set has only relative value. Although some cases do not conform, it may be said that the end of cleavage is reached with the establishment of a definite size relation between nucleus and cytoplasm (the nucleo-cytoplasmic ratio), the particular ratio being that which is normal for that particular stage of that particular species. It is of course to be noted that where differentiations due to yolk, etc., are present there cannot be a single nucleo—cytoplasmic ratio for an entire embryo. It is perhaps better to say that cleavage is ended when a typical blastula is formed consisting of the first primary germ layer with the cells arranged in an epithelial layer.

It must be borne in mind by the analytical student of developmental phenomena. that three distinct kinds of processes are involved in the transformation of the egg into a larva. These are cell division, growth, and difi‘erentia.tion—processes which are not only distinct, but to a certain extent antagonistic, the last two never taking place at the same time with the first, and only to a minor extent together. (Differentiation is here used in the sense of histological differentiation, for of course in another sense its processes take place by means of cell divisions.) We may regard the actual growth processes, that is, increase in bulk, as anabolic, and obviously cell division is largely katabolic, from which it follows that the two cannot take place as dominant factors in the same cell at one time. Their independence in this relation does not mean, however, that they are not dependent on each other, for if they do not maintain their proper balance further development on the part of the organism as a whole cannot occur.

Before discussing the types of cleavage it should be pointed out that development is possible in some cases without regular cleavage. This really amounts to a delay in cell division as contrasted with nuclear division. Several nuclear divisions take place in succession without the cleavage of the cell body; at a later stage the cell areas are cut off around the nuclei all at one time. Examples of such development are found in the alcyonarian Clavularia (Kowalevsky and Marion), Ii’em'lla (Wilson), Alcyonium (Hickson), Tealia (Appellof), and Cucumaria glacialis (Mortensen).

It should also be made clear that a classification of cleavage types has nothing of the evolutionary significance which the systematic grouping of animals has, nor can it be made a basis for systematic classification. Although cleavage types are in general stable and are coextensive with certain animal groups, yet there are very notable departures in this regard. It will be seen by an inspection of the table below that the cephalopods have a different type of cleavage from other molluscs, that the scorpions cleave differently from others of their class, and that many other exceptions also exist. Indeed, there are instances in which nearly related species have different methods of cleavage, and the extreme modification of this sort is perhaps seen where two types of cleavage occur in the very same species. An example of this last case is seen in Polyphemus oculus, whose summer eggs undergo regular holoblastie cleavage, whereas in the winter eggs a type of Ineroblastic cleavage occurs which strongly suggests conditions found in the superficial cleavage of some insect eggs. These are, of course, extreme cases which merely serve to show how variable cleavage forms may be, doubtless as adaptive responses to the conditions of development, without having any special phylogenetic or taxonomic significance.

1. Classification of Egg Types

On the basis of their structure, particularly as modified by the amount of yolk present, Balfour divided eggs into three groups, alecithal, telolecithal, and centrolecithal.

Alecithal, or better homolecithal, eggs (also called isolecithal) are those in which there is relatively a small amount of deutoplasmic material uniformly distributed throughout the egg. Such eggs cleave regularly into equal-sized blastomeres. Echinoderm eggs are examples.

Telolecz'thal’eggs have a clear axial structure, the protoplasm at the animal pole containing very little yolk, while the vegetative half of the egg is rich in yolk. The frog egg may be taken as an example. The blastomeres at the vegetal pole, therefore, are distinctly larger by reason of their large yolk content, and the cleavage cavity is pushed toward the animal pole. Such eggs necessarily have unequal cleavage, if the furrows are able to cleave the yolk at all.

In another type of telolecithal eggs the accumulation of yolk is so great that it cannot undergo cleavage. The protoplasm is thus limited by the yolk to a small disc at one pole of the egg while the yolk fills the rest of the space. The eggs of birds and of teleost fishes furnish excellent examples. When cleavage takes place it cuts through the disc only, and is therefore discoidal.

Telolecithal eggs are thus of two sorts: holoblastic, those in which the entire egg cleaves, although unequally; and meroblastic, in which only part of the egg mass can be divided, and that by either discoidal or superficial cleavage. The terms “partial” and “total” cleavage also apply to these types. Between these two types there are of course intermediate conditions shown by various eggs. These two extremes lead to two types of embryo formation; in the holoblastic egg the entire egg goes to form the embryo body, but in the meroblastic type the germ disc is all that is involved, and the yolk is in the form of an appendage, the yolk sac.

Centrolecithal eggs are richly yolk laden. The cleavage nucleus in a cytoplasmic area is located at the center of the egg surrounded by the yolk. A protoplasmic layer, however, covers the surface and is connected to the central mass by fine plasmic threads. Some centrolecithal eggs are so definitely oriented that the position in which they are laid indicates exactly the axes of the future embryo, but in general the primary axes are not well developed in eggs of this type, although bilaterality is to be noted. Cleavage begins by the plasma island in the center multiplying through several nuclear divisions, thus forming a syncytium. These “blastomeres” now move radially toward the periphery and unite with the plasma layer which is at once cut up into cytoplasmic areas of similar size, each supplied with a nucleus. The central yolk does not divide, or if division starts it is incomplete. This constitutes superficial cleavage, the blastoderm forming about the yolk which fills entirely the cleavage cavity (if one might use that expression).

2. Classification of Cleavage Types

Dependent upon the amount of yolk they contain, the first divisions of eggs are total or partial. Eggs having total cleavage are said to be holoblastic; those having partial cleavage are meroblastic. It is also customary to classify types upon the basis of the planes of symmetry which are to be observed in the dividing egg. Thus in radial cleavage there is uniform distribution around the egg axis, but in disymmetrical cleavage there are two centers of symmetry, and in bilateral cleavage a single plane only will cut the egg in equal halves. In spiral cleavage the plane of the spindle"in the first divisions turns in a spiral with reference to the egg axis. These four cleavage types are holoblastic. Meroblastic eggs may be divided into superficial and discoidal. In the former case the blastodisc forms around the outside of the egg but involves practically the entire surface whereas in the latter only a small disc undergoes segmentation, owing to the extensive amount of yolk present. These relationships are shown by the accompanying table as are the animal groups to which they belong» We shall consider these types in order.

EGG GERM C LEAVAG E STRUCTURE CLEAVAGE FORMATION TYPEs EXAMPLES

Porifera 1. Rmlial Cnidaria Echinoderma

Equal 2. Disymmetrical Ctenophorcs

Nematodes Rotifers Homolocithal Holoblast ic Ascidians

3. Bilateral Amphioxus Petromyzontidae Amphibia Incquul Higher mammals

Polyclads Nemertcans

[4, Spiral Annelids and moll11.~cs(ox<'(-pt(~(-pl1'l‘eloleci1lml alopods)

Scorpion Cephalopods Pyrosomcs

[ ‘ Myxinoids

[ Discoidal Elasmobranchs T eleosts Gymnophionans Reptiles

Birds Monotremes

5. Discoidal

Meroblastic

Arthropoda (except scorpions) Controlecitlml Superficial U}. Superficial Someotherscattered ' forms among coelenterates 26 EGG AND CLEAVAGE TYPES

3. Classification on the Basis of Predelineation

Before considering the details of these different modes of cleavage it is desirable to mention certain qualities of eggs by which they are related to the later developing stages. With reference to the degree of predelineation shown, Conklin has recognized two types of eggs as follows:

a. Eggs with Determinative Cleavage. The best examples are the eggs of annelids, mo1‘uscs (except the cephalopods), and the ascidians. In eggs of these groups after fertilization, but before cleavage, there can be recognized in the one-cell stages regions or definite areas in the egg protoplasm, the organ-forming regions of His. These regions are distinguishable in various eggs by different means, among which are differences in yolk content, presence or absence of pigment, clear or granular condition of the cytoplasm, and the presence of very fine granules which can be demonstrated by definite staining methods. The centrifuge has also proved useful in detecting different substances in the egg which as cleavage advances become segregated into different blastorneres. (Cf. the work of Lyon, Lillie, Conklin, etc.) The study of the cell lineage of such an egg shows that these blastomeres are the beginnings of definite organs of the embryo. In other words the egg substances which are thus localized into regions are determined for the production of definite organs of the later stages. During the progress of the investigations which established the determinative character of certain eggs there was a great deal of discussion, and even controversy, as to whether blastemeres were totipotent, that is, interchangeable, or constituted a mosaic work, each being capable of taking only its own special place. We now understand that eggs which have determinative cleavage have an operative mechanism of great precision, and that correspondingly they have lost much of their power of regulation. Because of their content of organforming substances they are apparently destined for a particular fate, and can undergo very little regulation. These statements of fact for determinative eggs do not apply to eggs of the next type.

b. Eggs with Indeterminate Cleavage. The problem of determination, after all, is a problem of differentiation. In eggs of this indeterminative type differentiation sets in only relatively late after the embryo consists of many cells. In the preceding case the marks of differentiation are recognizable from the very first. In indeterminative eggs, there are no orgamforming regions, little histological differentiation is found, and the blastomeres appear to be alike. Such eggs are simple in structure and are accompanied by high capacity for regulation. Examples are: vertebrates, some insects, most arthropods, cephalopods, most eehinoderms. In them the power of regulation is much greater than in the determinative type, and there is much evidence to indicate that the fate of a blastomere is often a function of its position, or that the blastomeres are totipotent and without special “prospective significance.” In Part Two the chapter on the determination problem gives the evidence in more detail.

Bibliographic Note

Among the more important accounts of the subjects contained in this chapter are the following: Korsehelt and Heider, Conklin. Morgan in “Experimental Embryology,” Cowdry’s “General Cytology” (especially the article by Conklin on "Cellular Difierentiation”). These works are cited in full in the bibliography on page 406.

Cite this page: Hill, M.A. (2024, April 20) Embryology Book - Outline of Comparative Embryology 1-3. Retrieved from https://embryology.med.unsw.edu.au/embryology/index.php/Book_-_Outline_of_Comparative_Embryology_1-3

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