A textbook of general embryology (1913) 6

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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!

Kellicott WE. A Textbook of General Embryology (1913) Henry Holt and Co., New York.

Kellicott (1913): 1 Ontogeny | 2 The cell and cell division | 3 The germ cells and their formation | 4 Maturation | 5 Fertilization | 6 Cleavage | 7 The germ cells and the processes of differentiation, heredity, and sex determination | 8 The blastxtla, gastrula, and germ layers. Morphogenetic processes

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Chapter VI. Cleavage

The grosser and externally visible processes of development begin with the cleavage of the fertilized ovum, or zygote. The period of cleavage therefore may be regarded as the second of the "grand periods" in individual history. During the first general period occur all the events leading up to and including the final establishment of the zygote, a single cell, but a new organism. During this second period the Metazoan really becomes made up of many cells.

The essential process underlying many of the varied phenomena of cleavage is a process already familiar, mitotic cell division; but it is true that cell division continues long after the cleavage period proper is terminated, in some tissues throughout the life of the organism. And as we shall soon see, the process of cleavage involves a great deal more than merely a succession of cell divisions.

Certain general characteristics of the mitoses of the period of cleavage, or segmentation, of the zygote, may be observed, but it is difficult to state precisely wherein these cell divisions differ from those of later development. Probably the most significant characteristic of the divisions of this period is that they are rarely at random, but nearly always occur in an orderly fashion, according to a definite schema or plan, which is quite fixed for each species or larger group, and which involves the entire cell conmiunity. The mitoses of cleavage are frequently very unequal and the daughter cells may be very unlike, not only in size, but further as regards cytoplasmic character and the nature of various cell inclusions, which may be distributed dissimilarly during these divisions. After the first few mitoses the blastomeres may not divide synchronously, so that the regular and rhythmic geometric increase in their number to 2, 4, 8, 16, etc., is very rarely continued after eight or sixteen cells have been formed; the regularity of division may be disturbed as early as the second cleavage. In some forms (e.g., Echinoderms, Godlewski) the nuclei of the daughter cells enlarge considerably after each division, in some cases perhaps even to the original size: the cell bodies fail to do so. The result is the constant increase in the relative size of the cell nuclei; in other words, while the amount of cytoplasm increases only slightly or not at all during cleavage, the amount of nucleo

Fia. 105.— Types of blastulie. A. AmpUoxua (cceloblaBtulii). B. Petromtpon. After von Kupffer. C. Nolurua (TeleoBt) (diBcoblaatula). D. Clava (Hydroid), After Horgitt. (Solid type.) a, aniiuBl pole; c, segmenUtloil cavity or blaatoocel ; p, periblast (a uon-cellular protoplaamio layei restins upoQ the yolk mass); c, vegetal pole.

plasm increases considerably, so that at the close of this period the organism contains an appreciably greater proportion of nuclear material than did the zygote. This, however, may not be regarded as a general characteristic of the cleavage mitoses in all organisms (Conklin).

After a number of cells, varying in different species, have been formed they become arranged so as to limit an internal cavity filled with a fiuid. In its simplest and apparently most primitive or typical form, the figure thus established is a hollow sphere, the wall of which is composed of a single layer of cells or blastomeres. This structure, however its actual form may deviate from this type, is termed the blastula, and the cavity within is the bldstoccelj or segmentation cavity (Fig. 105). The diverse forms of the blastula depend immediately upon the arrangement of the preceding cell divisions; the blastula may in some cases be almost or quite solid, so that the blastoccel exists only virtually.

About the time the blastula is formed the successive cleavages have reduced the cell size to a physiological minimum and thereafter the daughter cells increase in size subsequently to each division, and there is no further reduction in the size of the blastomeres; the volume of the cytoplasm, as well as of the nucleoplasm, commences to increase, in other words the organism begins to grow. The relative time of the appearance of this growth phase is widely diflferent in different forms; in the Echinoids it appears when about sixty-four cells have been formed (Godlewski). While there is no general and externally visible indication marking a definite close of the cleavage period, the formation of some type of blastula, or the initiation of cytoplasmic growth, is more or less arbitrarily assumed to mark its termination, although many of the processes characteristic of this phase of development, including of course cell division, may continue for some time longer. We may now define cleavage as that early period of development characterized externally by a rapid and orderly succession of mitoses, which results in the formation, from the zygote, of a regularly arranged group of small blastomeres possessing relatively large nuclei.

In most cases there is a marked tendency for the blastomeres to assume a spheroidal form, more or less modified by the tension with which the cells are held together, by the pressure of the egg membranes, etc. Sometimes the cells round up so as to become practically spheres, in contact with one another by greatly restricted surfaces (Amphioxus, Echinoderms, Ccelenterates). In other instances (frog, chick. Arthropods) the separations between the blastomeres are mere grooves or furrows on the surface of the mass; here the cells are broadly in contact and in some instances remain connected by very delicate protoplasmic bridges similar to those connecting tissue cells previously mentioned. In a few instances (some Arthropods, a few Coelenterates) these early cell divisions may be imperfect, the nuclei alone dividing and forming a syncytium; later the cytoplasm also divides simultaneously into a number of complete cells. Or cells once fonned may fuse into syncytial masses, as in some Crustacea.

The internal processes of development occurring during this period are of greater importance than the external phenomena. As stated above (Chapter V) one of the underlying processes of great importance seems to be the synthesis of chromatin which occurs at this time.

In the opinion of many this is a highly characteristic chemical process of early development. It results from rapid oxidations within the egg which were made possible by the transformation of the egg membrane from a condition of relative impermeability, to a state of high permeability, oxygen thus being readily admitted from without. This transformation is thought to result from the chemical reactions of the cytoplasm following fertilization, during which there occurs also the activation, or perhaps the introduction, of specific enzymes which bring about this characteristic oxidative synthesis.

This view as to the chemical process most essential in cleavage agrees well with the "kern-plasma" theory of Richard Hertwig, according to which, as already mentioned, the ovum and zygote are to be regarded as instancing abnormal or especially adapted relations between nucleus and cytoplasm; for here the relative amount of cytoplasm is far in excess of the common proportion. In cleavage, with its proportional increase in nuclear substance, we should see a restoration to or toward


the normal of the kern-plasma ratio =.^-1 ( ?7^)

Volume \Kc/


Another internal process is of prime importance. We have already become familiar with certain facts — that one result of maturation and fertilization is the presence in the zygote of two similar chromosome groups, derived respectively from the male and female parents; that while the egg nucleus nearly always returns to a "resting" stage before its fusion with the sperm nucleus, the latter may or may not do likewise before the fusion of the two germ nuclei into the cleavage nucleus; that the egg and sperm nuclei may or may not form separate spiremes each with ^ chromosomes during the prophase of the first cleavage figure of the zygote; and finally that whatever the preliminary details may have been, the constant and essential facts regarding this first cleavage figure are, (1) that the chromosome group now consists of the full somatic number of univalent elements, (2) that these are present in pairs, and (3) that they have been derived equally from the two parents.

As cleavage begins the first important step is the longitudinal division of each chromosome; the halves diverge during the anaphase of the first mitosis, and into the nucleus of each daughter cell there passes a precisely similar group of s chromosomes, paired as before and derived in equal numbers from each of the two parents. In each succeeding mitosis the same thing happens. So that in every cell of the blastula, and probably even of the fully matured organism, the nucleus is composed of substance derived in equal parts from the mak and the female parents (Fig. 106). In some forms (Copepods, Ascaris) the two parental chromosome groups appear to remain fairly distinct lip to a late stage in cleavage (Fig. 107). And in some cases of hybridization, when the chromosomes of the two parents are easily distinguished by differences in size and form (e.g., the hybrids of FunduLus and Menidia described by Moenkhaus), such a process of equal distribution of the chromosomes can be clearly followed into the blastula stage (Fig. 39). It may fairly be assumed, therefore, that Huxley's comparison of the body of an organism with a web, of which the warp comes from one parent, the woof from another, has been justified by the subsequently discovered facts of development. This idea has been spoken of as the " autonomy of the male and female chromosome groups." It follows from this that the parental chromosome groups of the primordial germ cells of the new organism are also separate and remain so through their descendants, the oOgonia and oocytes, or spermatogonia and spennatocytes, of the mature individuals, until their period of synapsis, when the members of each pair of chromosomes, similar but of diverse ancestry, unite forming a single bivalent chromosome which is represented in the mature ovum or spermatozodn finally formed. It has already been suggested that this process of synapsis, the ultimate fusion of paternal and maternal chromoaomes, may be regarded as the final step in syngamy, and that it is at the same time the fiirst step in the beginning of a new organism.

Fig. 100. — DiHgrams iUuBtratilig the distribution of the paternal and maternal chromosomes during cleavage. A. Zygote containing sperm, d'. and egg, 0, ptoDuclei, with BimilBr chromosome groups. B. First aleavage figure. All the cbromosomeB on the spindle, and each divided. C Two-cell stage, each nucleui containing equivalent chromosome Eroups of psterDol and maternal origin. D. Second clesTage figure: the first figure is repeated in each cell. E, Four-cell stage. Nuclei all alike, and each composed lit oimilaf contributions from each

Fig. 107. — A. Cleavage figure in one of the first two blaatomeres of the egg of the Crustacean, Cydops ilrenutM, showing the independence of the pstemai and tnatemal ohromosome groups. After RDclcert. B. C, D. Primitive germ cells from embiyOH of the skate, Raja, showing the duplex character of the nuclei. B and C are from a stage about the close of gastrulation ; D from a larva of 10 mm. After Beard.

With these briefly stated introductory facts in mind we may proceed to a more precise description of the events of the cleavage period. The process of cleavage may be described in several different ways, or rather from several different viewpoints. We may group these all under two heads and consider cleavage, first, as a morphological process, emphasizing primarily the/ofTus of cleavage and describing the relation of the cleavage planes and the blastomeres (a) to the entire zygote, and (b) to each other. Then second, we may emphasize chiefly the physiological aspects of cleavage describing the relation of the cleavage processes (a) to the structure or organization of the ovum and zygote, (6) to the later stages in the development of the mature organism. As a matter of fact these aspects of cleavage are not really separate, for all the particular phenomena of cleavage, as of development in general, are to be referred to a single fundamental condition, namely, the organization of the ovum or zygote as it is related to external conditions; and if our knowledge were complete here we should be able to describe all the phenomena of cleavage from a single viewpoint. But for the present we shall find it more convenient as well as more instructive to separate more or less arbitrarily and to consider apart, the chiefly morphological and the chiefly physiological aspects of this process.

In considering the relation of cleavage to the grosser structure of the zygote we find that one of the primary factors in determining the form of cleavage is the relative amount and the form of distribution of the yolk and other deutoplasmic substances contained in the ovum. In Chapter III, three types of eggs were described on this basis : (1) homolecithal ov isolecithal (alecithal), containing little deutoplasm, distributed with considerable uniformity throughout nearly the entire ovum: (2) telolecithal, containing var3dng, often considerable amounts of deutoplasm chiefly localized toward the vegetative pole of the ovum; (3) centrolecithal, really a form of telolecithal ova in which the deutoplasm has a central rather than a polar localization.

Corresponding in a general way with these variations in yolk distribution we may distinguish certain types of cleavage, each however with certain variations which may soinetimes appear as connecting intergradations. First we may distinguish complete and incomplete cleavage. When the eggs are comparatively small and of the homolecithal type, the earlier cleavage planes pass completely through them in meridional and latitudinal planes. Theoretically the simplest form of complete cleavage is that known as eqiml cleavage, where the egg and the blastomeres formed from it are always divided equally, so that the constant result is a group of similar cells. This is rarely if ever completely realized; the nearest approach to it is seen in the Holothurian, Sj/nap^a (Fig. 108). In most examples of so-called equal cleavage slight inequalities may be detected even as early as the two-cell stage (Amphioxus), and quite frequently in the four-, or eight-cell stages. This modification of equal cleavage is known as adequal. Amphioxus and some of the Echinoderms (Fig. 109) illustrate the fact that no sharp distinction can be drawn between equal and unequal cleavage, for equal cleavage soon becomes unequal, and the transition appears gradually.

More frequently the cleavage though still total is distinctly unequal, at least by the time eight cells are formed, and often from the very beginning of cleavage. This is characteristic of those telolecithal eggs in which the accumulation of yolk is slightly or moderately marked, as in most of the Platyhelminthes, Nemathelminthes, Annulata, Trochelminthes, MoUusca, Ganoids, and Amphibia (Figs. 110, 111). This leads to a second general type of cleavage, the incomplete type, where a portion of the ovum remains uncut by the cleavage planes (Fig. 113). Such eggs are known in general as merohlasticy in distinction from the holoblastic ova whose cleavage is complete. In telolecithal eggs with very large accumulations of deutoplasm, the cleavage planes are nearly restricted to the protoplasmic region and extend only a short distance out into the yolk; this is known as partial cleavage. When the protoplasmic part is quite definitely restricted to the animal pole, cleavage is of an extremely incomplete type known as discoid (Fig. 116), and the result is the formation of a small cap or disc of cells on the surface of the yolk mass (Teleosts, Reptiles, Birds). Or, if the egg is of the centrolecithal type, cleavage is limited to the peripheral protoplasmic layer and is known as superficial (most Arthropods) (Figs. 117, 118).

Since there are all intermediate conditions between homo-, telo-, and centrolecithal types of ova, we find, as we should expect, all corresponding intermediate conditions between these various forms of cleavage; the details are not particularly instructive and may be omitted.

We may now turn to the morphological description of the relations of the blastomeres among themselves. Before describing the various relations which these may exhibit, it will be useful to describe briefly a simple form of total and equal cleavage, which may be regarded as a typical form. Such a form of cleavage is indeed rare but it is found in the homolecithal and holoblastic egg of the sea-cucumber, Siptapta, as described by Selenka (Fig. 108). The earlier cleavage planes always appear in a definite relation to the polar structure of the ovum, and they are described as if the main axis of the egg were in a vertical position. The first cleavage plane passes through both poles and the chief axis of the egg, dividing it equally and appearing on the surface as a complete meridian. The second plane is similarly meridional or vertical, is at right angles to the first, and divides the egg into four equal quadrants. The third division plane is at right angles to the first two and is therefore horizontal. In Synapta it is practically midway between the poles of the egg and is therefore described as equatorial. In most ova cleaving according to this general rule, the third plane is displaced a variable distance above the equator and is then termed latitudinal. When equatorial this cleavage divides the four equal cells into eight, again equal, arranged in upper and lower groups of four, known as the upper and lower quartets. -The fourth cleavage is again meridional and is really double, for two planes appear simultaneoudy dividing each pair of opposites in each quartet similarly; this results in the formation of upper and lower octets. The fifth cleavage is horizontal or latitudinal, and is again double for it divides simultaneously the upper and lower octets each into two horizontal groups of eight cells, so that the ovum is now divided into eight vertical rows of four cells each. The cleavages continue to alternate meridionally aud vertically, until about the ninth cleavage when 512 cells are formed. After this, and in fact usually before this time, the synchronism of cleavage begins to be disturbed, some of the cells dividing more rapidly.

Fig. 108. — Cleavage in the Bolothurian, St/napla, SlighUy TCheiDBtued. From Wilson, "Cell." after Seleoka. A-E. Two-, foui^, eight-, Biiteen-. and thirty-two cell staies. F. Blaatula of 128 c^a. B, in polar view, others in sido

One of the very frequent causes of departure from this simple schema is the telolecithal character of the ovum. Here the upper quartet is usually smaller than the lower and the fourth cleavage appears earlier in the upper quartet or cells of the animal pole. This leads very soon to an irregularity in the rhythm of cleavage, which may be entirely lost after eight or sixteen cells are formed.

This typical outline of cleavage serves as a basis to illustrate certain "laws" of cleavage which may be referred to briefly at this point, although their applicability is now known to be very limited. The first of these is the Sachs-Hertwig law describing the geometric relations of the successive cleavage planes. This law really consists of two parts which may be stated as follows: (1) The nucleus of a blastomere (or of any cell) tends to assume a position near the center of the jrrotopUismic mass. From this results the equal division of the cell, provided it is free from deutoplasm, or its unequal division if the cell contains deutoplasm not uniformly distributed, for in the latter case the center of the protoplasmic mass does not correspond with the center of the entire cell. (2) The chief axis of the mitotic figure tends to lie in the longest axis of the protoplasmic mass. The result of this is that in cells that are approximately spherical and homogeneous with respect to yolk content, successive cleavage planes tend to alternate at right angles with one another, for it would always be the longest axis of the cell that is divided, and in most cases any other axis would be greater than one-half the longest and no two successive spindles would be parallel. The regular alternation of cleavage planes probably depends, as a matter of fact, upon a more fundamental relation, namely, the position of the centrosome. At the conclusion of a mitosis the centrosome lies at one end of the axis passing perpendicularly to the plane of division; when the centrosome divides, its halves usually migrate symmetrically to opposite sides of the nucleus, occupying the poles of an axis Ijdng parallel with the plane of the preceding division, and since division always occurs at right angles to the axis connecting the centrosomes, the plane of one division will be at right angles to that of the preceding or succeeding cleavage (Fig. 24). Any other relation between successive cleavage planes involves either a change in the relative position of the centrosomes, or a rotation of the cleavage spindle after its formation. Thus in the formation of a simple epithelium, where successive cleavages are nearly parallel, the centrosomes migrate through approximately 90° at some time during the interkinesis; and in the cleavage of some ova encased in comparatively rigid shells, the position of the spindle may change {LepaSf Bigelow).

Balfour's law of cleavage, which is really a corollary of the first part of the Sachs-Hertwig laws, concerns the rate rather than the geometrical relations of cleavage. This law states that the rate of cleavage is inversely proportional to the amount of deutoplasm contained within the cell. It follows from the fact that the nucleus tends to lie in the center of the protoplasmic mass, that in the unequal division of cells containing localized deutoplasm, the smaller cell will contain relatively a smaller proportion of yolk than the larger cell, and consequently, being free from the influence of the dead and inert deutoplasm, will be able to divide sooner.

While these laws are often applicable to the processes of cleavage in a general way, the exact study of cleavage in a great variety of forms has disclosed very numerous exceptions and contradictions. On the whole we may say that such laws, though still retaining a limited applicability, are chiefly interesting as indicating the attempt to refer the phenomena of cleavage to the grosser mechanical relations of cell structures. It is now clear, as we shall . see later, that other factors are of greater importance in determining the form and rhythms of cleavage. The fundamental "organization" of the ovum, which is not only morphological but physiological as well, is the primary factor in determining the characteristics of cleavage. The numerous exceptions" to these laws of cleavage are definitely related to both the organization of the ovum and also to the structural and functional characters of the later stages of development, since these too are primarily determined by the same organization factor.

Most of the conditions which form exceptions to these rules, and are therefore deviations from the simple and regular form of cleavage like that of Syna'pta, may, following Wilson (The Cell," etc.), be grouped under three heads. (1) Unequal Division, While this is usually related to differences in deutoplasmic content, there are many instances where no such relation can be made out and the inequalities must be explained upon other grounds (e.gr., the micromeres of the Echinoids, the teloblasts of certain Annelids and Molluscs). (2) Cell Displacement. This may result from the atypical position of the spindle or from the shifting of blastomeres after they have been formed. Often the individual blastomeres are only lightly held together since they normally show a tendency, often very marked, to assume a spherical form. Under these conditions they might tend to assume a position described by the law (Plateau's) of "least surfaces" or "minimal contact," according to which a group of elastic spheres, like bubbles, held together and yet free to move, tend to become arranged in such a way as to reduce their exposed surfaces to a minimum. But there are frequent exceptions here as in the case of the other "laws" of cleavage. Furthermore, the active migration of blastomeres is not infrequent, so that cells may ultimately be found in regions considerably removed from the place of their formation (Rotifers, Molluscs). (3) Rhythm. The rate of division frequently does not correspond with the relative amount of deutoplasm. The factors regulating the rhythm of division still remain largely unknown. It is true here as in many other "exceptional" cleavage phenomena, that the deviation is related to the future morphological or functional character of the developing organism or of parts of it. We shall return to this aspect of cleavage later.

With these general considerations in mind we may proceed now to a more exact description and classification of the geometric forms of cleavage. Here we shall find illustrations of many of the preceding statements. Considering first the various forms of complete cleavage (holoblastic ova), we may distinguish rather roughly, four types, radial, spiral, bikUeral, irregular.

(1) Radial. — This is the form exemplified by Synapta (Fig. 108), already described as being geometrically the simplest. This should perhaps better be termed rotaiorial than radial, for while the blastomeres are arranged in symmetrical fashion in any single plane perpendicular to the main axis of the egg, there are usually considerable differences in size between the cells of the animal and vegetal poles. Cleavage of this type is found in the sponges, jelly-fishes, and many Echinoderms, in some Nematodes and Rotifers. In the sea-urchins (Fig. 109) the third cleavage is meridional in the upper quartet, in the lower latitudinal and very unequal, cutting off a quartet of very small cells or micromeres which curiously are found at the lower or vegetative pole.

(2) Spiral, — This may be regarded as a modification of the radial type resulting from the displacement of cells so that the blastomeres above and below any horizontal cleavage furrow tend to alternate with one another in a vertical direction, somewhat like the bricks in a wall or the bones of the wrist. They may be cut off in this way on account of the obliquity of the spindle in the parent cell, or they may shift to this position after having been formed according to the radial plan. This spiral arrangement may be foreshadowed in the four-cell stage by the meeting of the first two cleavage planes at the poles of the egg in the form of a zig-zag line instead of at a common point.

Pig. 109.— Cleavage in the Bea-urchin. Strorisvlocenlrottn Undue. From Jenkinson, after Boveri. Animal pole uppermost in all caaes. a. Primary oocyte Burtounded by jelly, and containing large germinal vesicle with nucleolus. Pigment uniformly distributed over aurface. b. Ovum after formation of poiar bodies. Pigment forms a band below the equator, c, d. First cleavage, e. Eight-cells. Pigment almost wholly in lower quartet {vegetative blaatomerea) . /. Siiteen-cells. The lower quartet has divided latitudinally and unequally, foirning four micromeres at the vegetal pole; the upper quartet has divided m&ridionally forming a plate of eight cells, g. Section through blastula. h. Later blaatula, showing formation of mesenchyme at lower pole, i, j, k. Three stages in gastrulation, showing the infolding of the pigmented cella to form the endoderm (archenteron). In j the primary mesenchyme is separated into two masses, in each of which a spicule is formed (ib). In k the secondary, or pigmented, mesen* chyme is being budded off from the inner end of the arcbenteroiL

Fig. 110. — Cleavage in the Annulate, Polygordius. From Wilson, "Cell.** X, B, Four- and eight-cell stages, from the animal pole. C. Side view of eightcell stage. D. Side view of sizteen-cell stage.

One of the simplest illustrations of this type is the adequal cleavage of Polygordius (Fig. 110) but it is also well represented by the markedly unequal cleavage of many Platyhelminthes, Nemertines, Annelids, and Molluscs (Figs. Ill, 112, 119). These forms illustrate at the same time a graduated series in the inequality of the blastomeres. This inequality may appear in different stages; in the very first division of the egg {Nereis); at the second (Clavelina); third {Cerebratulus, Fig. 112); fourth (Template:Sea-urchin), or still later (Synapta). The direction which the spiral takes is fixed in each species; it is described as dextral {deociotropic) or sinistral (Iceotropic) when the upper cells are rotated clockwise or counter-clockwise respectively, as viewed from the animal pole.

Fig. 111. — ^The eight-cell stage of four animals showing gradations in the inequality of the third cleavage, and in the extent of the spiral rotation of the micromeres. From Wilson, "Cell." All viewed from the animal pole. A, The leech CUpsine (Whitman). B. The chsetopod Rhynchdmis (Vejdovskj^), C The lamellibranch Unio (Lillie). D. Amphioxua,

(3) Bilateral. — In this form we see a second modification of the radial type, which is first indicated by the fact that the third of the meridional cleavages fails to reach precisely the poles of the egg but meets either the first or second plane at some distance from the pole. In this way it comes about that.

Fig. 112. — Cleavage in the Nemertean, CerfSbraivlus marginatua. From Korschelt and Heider, after Zeleny. X 216. A, B, Two- and four-cell stages in side view. C. Four-cells, from animal pole. D. Eight-cells from vegetal pole. E, Eight-cells, side view. F. Sixteen-cells, side view. G. Twenty-eight cells, side view. H, Twenty-eight-cells from vegetal pole. For explanation of lettering, see p. 248.

one of the first two cleavage planes, usually the first, becoiaes the plane of a bilateral symmetry which may remain quite pronounced for some time, and indeed corresponds with the median plane of the bilaterally symmetrical adult. The bilateral type of cleavage is found in the Cephalopods (Fig. 113), a few Rotifers and Nematodes, in Amphioxus and the Ascidians, and perhaps in most of the Craniates, but in these last forms variations are more frequent, especially in those forms with discoid cleavage.

Fig. 113. — Meroblastio cleavage in tha squid. Loligo pealii. A, B, Egg viewed obliquely, ahoiriiig animal pole. X 45. After Watas6. C, D. Surface views ot ammBl pole, more highly magnified, to show bilateral arraDgement of blastomeres. From Wilson. "Cell." after Wataa6. A. Four-cell atase. B. About Biity-oellB. Cells at animnl pole vety small, lowermoBt cells incomiJete, ceU walls extending down toward the uncleaved lower pole. C. Eight-cell stage. D. The fifth cleavage (sixteen to thirty-two cells), a-p, marks the plane of the first cleavage and the median plane of the organisni; l-r, marka the second cleavage, and the transverse plane of the organism.

A rather special form of bilateral cleavage known as the disymmetrical type is found in the Ctenophores. In these Coelenterates the first and second furrows are meridional, the planes are complete, and divide the egg adequally. The third, a double cleavage, is oblique to the first, passing in the same general direction as this, on each side of it, but approaching the vegetal pole more closely than the animal. The descendants of each of the first two cells then become symmetrically arranged about the second plane so that two similar groups of cells are formed, each group bilaterally symmetrical with reference to a plane perpendicular to the plane of symmetry of the entire cell group (Fig. 114).

Fig. 114. — Diagrammatic representation of the cleavage in the Ctenophore (based upon BeroS), After Ziegler. A, Four-cells, from side and above. B» Eight-cells in side view (the animal pole is downward here, and in D). C. Eight-cells, from animal pole. D, Sixteen-cells, from side. E. Sixteen-cells, from animal pole, m, m. Median plane; t, t, transverse plane.

(4) Irregular. — In many phyla, scattered forms are known in which cleavage adheres to no single or simple type and may truly be said to be irregular. This does not mean that no definite plan is followed, for each species follows a fixed rule; these cleavage forms are in this sense regular, but cannot be described in general terms. We cannot stop to describe any of these instances in detail. Irregular cleavage may be found among the Porifera, Ccelenterates, Platyhelminthes, Molluscoids, Enteropneusta, and Teleosts; a typical example is illustrated in Fig. 116.

Fig. 116. — Irregular cleavage in the Turbellarian, Meaoatomum ehrenbergu After Bresslau. X 700. A. Three-cell stage, in section. B. Four-cells becoming five. Side view. C. Seven-cell stage, from animal pole. D. Twelve^ cells. A. Macromere, giving rise to Ai and At in the seven-cell stage. B. First micromere, forming Bi and Bt in the five-cell stage. C. Second micromere formed in three-cell stage, and giving rise to Ci and Ct in the seven-cell stage.

The remaining forms of cleavage are grouped as incomplete, •Und are found among those species with markedly telolecithal or centrolecithal ova (meroblastic). Here little or no geometric regularity of cleavage pattern can be made out. We may add a few details concerning discoid and superficial cleavage to the brief statements made on a preceding page.

Fig. 116. — Cleavage in the sea-basa Serranua atranut From H V WilMtl. A. Surface view of blasto<lisc in two-cell stage B Vertical section through four-cell stage. C. Surface view of blastodiso of Biiteen cells. D Vertical section through aixteen-cell stage E Vertical section throagh late cleavag«  stage, cp., central periblast m p marginal penblast s c. aegmentation cavity (blaatoccel).

Discoid. — This is chiefly characteristic of the Craniata but it is found occasionally in the Arthropods (Scorpions). In the ova of these forms there is often a fairly definite demarcation between the protoplasmic and deutoplasmic portions (Elasmobranchs, Teleosts, Reptiles, Birds) and the cleavage planes are practically limited to the former region, known as the blasto~ disc (Figs. 48, 157-159). The early cleavages may be fairly regular, and approximate either radial or bilateral arrangements, and as far as the protoplasm alone is concerned, the cleavage is frequently equal. When the protoplasm forms merely a disc resting upon a large mass of yolk, it is obviously impossible to speak of meridional or latitudinal cleavages; hence the cleavages are described as vertical, either radial or circular, and horizontal. The vertical cleavages soon become connected below the surface of the ovum by horizontal planes, separating the lower surface of the protoplasm from the underlying yolk, and the peripheral circular cleavages similarly separate the protoplasm from the outlying yolk (Figs. 116, 158, A). This is seen in the Teleosts, and in many Elasmobranchs. Reptiles, and Birds. After the protoplasmic blastodisc is divided into a number of cells, that is after it becomes a blastoderm, other cleavages may occur parallel with the surface, forming internal cells not visible upon the surface (Fig. 116), and the blastoderm may thus come to be many cells in thickness (Figs. 158, 105, D).

Some interesting transitions are to be found between total unequal cleavage and discoid cleavage, in those telolecithal eggs where the accumulation of yolk is not as great as it is in the Elasmobranchs, Teleosts, and some of the higher Craniates. Thus in the ganoid, Amia, and some of the Urodeles, as well as in the squid (Loligo), while cleavage is at first limited to the upper or animal pole, the earlier cleavages gradually extend down through the yolk mass and may finally divide it into a few large cells. Here the more peripheral circular cleavages (latitudinal) do not form any sharp separation between protoplasm and deutoplasm, and the yolk mass is for a long time only partially divided by meridional cleavages alone (Fig. 113).

Superficial. — ^This type of cleavage is characteristic of Arthropods in general and occurs elsewhere only in a few Coelenterates. The central accumulation of the yolk, as it occurs in these forms, is an unusual condition, and correlated with this we find several imusual features in cleavage. Of course in such an arrangement the most obviouft distinction between animal and vegetal poles is entirely lacking, and usually the position of the polar bodies and the external form of the egg are the only outward indications of the polarity of the ovum. Before cleavage begins the nuclear structures are located centrally, together with a small amount of protoplasm, and surrounding this is a dense mass of yolk, interpenetrated by a very fine network of protoplasm. At first nuclear division is not followed by division of the inert remainder of the e^ mass. But after, a varying number of nuclear divisions, the daughter nuclei separate, each accompanied by a small mass of protoplasm, the "protoplasmic ialand," and migrate to the siu^ace of the egg, continuing to multiply as they go (Figs- 117, 118). In this way the nuclear and cytoplasmic portions form a kind of superficial syncytium leaving the condensed and undivided yolk centrally.

Fig. 117. — Superficial cleavage in the Decapod. Dromia, (Sections.) After Cano. A, B. Intravitelline diviaionB of the nucleus. C, D. B. Formation of yolk-pyramida. F. Blastula; a euperficial layer of cells encloaing a mass of yolk, n, nuclei: Vi yolk pyramids; y, yolk bodies.

Now either of two things may occur. In some forms (Decapods, Copepods, Ostracods, Amphipods) cleavages appear almost simultaneously, dividing the egg completely into a number of cone-shaped cells with the apices directed centrally; these cells are known as yolU pyramids (Fig. 117). In some cases the formation of the yolk pyramids does not occur simultaneously throughout the egg, but occurs earlier on that side of the egg which corresponds with the ventral surface of the embryo and larva. After a variable but considerable number of yolk p3a'amids are formed the planes of separation gradually disappear except in the superficial protoplasmic layer, which alone remains cellular, while the yolk again becomes a solid mass. Such a process as this is taken to mean that this type of cleavage may have been derived from the total adequal type.

Fig. 118. — Cleavage in the beetle, Hydrophilus, From Korschelt and Heider, after Heider. A, B. Intravitelline divisions of the nucleus. C. Beginning of the formation of a superficial layer of cells. D, Later stage in formation of "blastoderm," or superficial cell layer. 6, blastoderm, or superficial layer of cells; d, yolk;/, nuclei surrounded by protoplasm (protoplasmic islands) ; z, nuclei remaining in yolk (merocytes).

In other examples of nearly all groups of Arthropods the yolk pjn-amids are incompletely developed or even entirely absent and the cleavage is strictly superficial (Fig. 118). Here, as in the preceding, blastomeres may be formed either wholly or only partially around the ovum. The preliminary divisions of the nucleus and the formation of protoplasmic islands occur as described above. In many of the Insects, whose cleavage is typically superficial, the substance of the ovum ultimately becomes completely divided internally.

The preceding classifications and descriptions of cleavage have been almost wholly morphological in their basis. But development is a process, not a succession of morphological stages, and there remains to be described the most important aspect of cleavage as a developmental process. We come then to still another classification of cleavage types as (1) deter-mindte and (2) indeterminate.

Cleavage is said to be determinate when exact morphological and physiological relations exist between the individual blastomeres and, (a) specific structures in the embryo and fully developed organism, and also (6) specific regions or substances in the ovum. Each of the products of cleavage is here a true organ, of particular and known value in development : blastomeres are not interchangeable, and their removal or destruction may lead to specific, related defects or abnormalities in later development {e.g., the Ascidians). Cleavage is described as indeterminate when the blastomeres seem to have no specific relation to the structure of either the egg or embryo and adult. Here all the blastomeres may have equal value as factors in development; they are more or less interchangeable, and removal or destruction leads only to the loss of a corresponding amount of substance, not to the absence of any specific or related parts {e.g., the Echinoderms).

In order that this classification should not be misleading, it should be said at once that this is a purely artificial distinction, based rather upon historical grounds than upon the facts of development, for these two types are completely connected by transitional conditions, and soon or late in development, all cells come to have specific, determined values.

Such a grouping as this, of the varieties of cleavage, obviously rests primarily upon a physiological rather than a morphological basis, for cleavage here is regarded as a process of development. In all cases of determinate cleavage the essential fact is that cleavage is not the mere division of the zygote into separate masses and units which can be moved about and moulded into the form of an embryo; cleavage is not merely a series of cell divisions, not the mere "vegetative reduplication of parts occurring in accordance with certain mechanical rules like those of Balfour, Sachs-Hertwig, or Plateau, mentioned above.

We have already seen that the exceptions to these rules are so numerous and so fundamental that they must have some real significance. It is now clear that in determinate cleavage aJ^ the details have a significance that is prospective, looking toward the structural and physiological characteristics of the larva or fully formed organism. It has been said regarding determinate cleavage that One can go over every detail of cleavage, and knowing the fate of the cells, can explain all the irregularities and peculiarities exhibited (Lilhe).

Why this should be true is partly explained when we remember that the characters of cleavage and of the fully developed organism are both the primary result of the underlying structure of the ovum. Cleavage stands as an intermediate process between egg organization and adult structure; it is one of the processes through which the primary organization of the ovum gains expression in adult form.

This view of the cleavage process is by no means the only, or the original view, but it serves to bring out clearly the fact that the problems as to the nature and causes of the differentiations occurring during the cleavage process are related to the problems of the nature and causes of the differentiations of adult structure. Indeed these differentiations have a common cause in the structure and reactions of the ovum, and are therefore fundamentally equivalent. In our introductory chapter we said that the organism is specific at every stage, the zygote, the group of blastomeres, the embryo, the adult, are all the same specific organism, and the question why the cleavage group is what it is, is the same as the question why the mature organism has its own specific and individual characteristics. The problems and processes of development are fundamentally alike throughout.

In continuing our discussion of this determinative aspect of cleavage we shall make little further attempt to distinguish the determinate and indeterminate forms since this separation is clearly artificial. The apparent differences between determinate and indeterminate cleavage may arise from the fact that one of the determining factors contains a variable. That is the time at which the organization of the egg becomes sufficiently complete to be effective in determining the course of differentiation of the blastomeres, may vary. Thus cleavage would be completely determinate if the egg organization were entirely or largely completed before the cleavage process begins, incompletely determinate if the organization is only partial, and indeterminate if the organization is only slightly marked during the earlier cleavages. For egg organization is progressive, it develops. So the determinate or indeterminate character of cleavage may depend, partly at least, upon the relative time during cleavage at which the organization becomes marked to such an extent as to determine the fate of particular blastomeres. Other factors obviously enter into the process and we shall review the subject from another point of view in the next chapter.

We have seen above that in nearly all species the earliest cleavage planes are definitely related to the polar axis of the ovum. The polarity of the egg is one of the fundamental aspects of its organization. We have seen also that the ovum often contains formed substances of various kinds, both protoplasmic and deutoplasmic, distributed in the cytoplasm in a definite and usually specific manner. It is a common feature of cleavage that the first plane symmetrically divides the egg or that part of it which takes part in the process of cleavage. And furthermore, with very few exceptions, this first cleavage plane coincides either precisely or approximately with the median plane of the embryo and adult. Nereis is one of the few exceptions to this rule; in this Annelid, as in some of the Urodeles, the second plane marks the future median plane.

The factors which appear immediately to determine the location of the first cleavage plane are mainly two. First and most important is the structure of the ovum itself, which in many cases, even in the unfertilized condition, is obviously bilaterally synmietrical ; the first cleavage plane corresponds closely with this plane of symmetry, which is therefore determined by the same *' organizational factor that determines the polarity and other structural features of the oviun. In other cases the symmetry of the egg appears to be radial or rotatoria! before fertilization and is converted into a bilaterally symmetrical structure by the entrance of the spermatozoon, the entrance path of which marks the plane of symmetry of the egg and developing organism, and determines the location of the first cleavage. It is quite possible, though hardly demonstrated as yet, that even in such cases there is really an invisible bilateral structure of the ovum which underlies the radial symmetry and really determines the point at which the sperm shall enter. In such a case the entrance path of the spermatozoon would itself be predetermined and could not be regarded as a primary factor in fixing the position of the first cleavage. This would obviously be the case in many of those eggs possessing micropyles. But in some eggs whose cleavage is indeterminate, even though they possess micropyles (Teleosts), there seems to be no regularity in the position of the first cleavage plane and no correspondence between this and any morphological characteristic of either ovum or adult.

The second cleavage, usually at right angles to the first, ordinarily corresponds with the median transverse a^ of the egg, embryo, and adult. The third cleavage is usually horizontal and separates animal and vegetal poles and corresponds most frequently with the separation, in the embryo and adult, of the more active animal, and less active vegetative tissues.

The facts that in all eggs of a given species or genus cleavage occurs according to a definite pattern, and that there may be an exact relation between the individual blastomeres of the cleaving ovum and the tissues and organs of the later organism, make it possible to speak of the "cell lineage (Wilson) of an organism. In forms with determinate cleavage it becomes possible to identify, even in a comparatively late embryonic stage, various groups of cells as the real lineal descendants of certain individual cells of the earlier cleavage group. In other words, it is in such cases possible to trace the structures of the embryo and adult back to single cells or parts of cells.

In order to illustrate the nature of the facts of cell lineage, and the completeness and exactness of the correspondence between blastomeres and differentiated groups of cells in the later embryo, we may describe briefly a typical instance, using as the subject one of the simpler and more regularly cleaving types, the Turbellarian, Planocera, as described by Surface. This account of the cleavage of this form should be read with the expectation of finding frequent "exceptions" to the laws of cleavage mentioned above.

In Planocera (Figs. 119, 120) cleavage is total, unequal, and spiral (dexiotropic).. The first plane is meridional and divides the egg into two adequal blastomeres known as AB and CD. The division of each of these is also meridional but is unequal, each forming a smaller and a larger cell, and dividing the entire ovum into two larger, and slightly unequal, cells known as B and D, and two smaller cells, also sUghtly unequal, known as A and C. Of these D is the largest, and from later development is known to be posterior in position; B is anterior, A on the left, and C on the right, as viewed from the animal pole and with reference to later structure.

The third cleavage is horizontal, unequal, and strongly spiral (dexiotropic). As usual among the Turbellaria the larger cells divide shortly before the smaller. On account of the size differences between the cells of the upper and lower quartets, we may describe the upper quartet of smaller cells (micromeres) as budded off from the lower quartet of macromeres. The quartets of micromeres are designated by small letters, the macromeres by capitals. Thus in the eight-cell stage we have the first quartet of micromeres, la, lb, Ic, Id, and the first quartet of macromeres, lA, IB, IC, ID. Successive quartets are designated by numerical coefficients, products of the division of the quartet cells by exponents. Thus in passing from eight cells to sixteen the macromeres bud off, this time in a Inotropic direction, a second quartet of micromeres, 2a, 2b, 2c, 2d, the macromeres themselves remaining known now as 2A, 2B, 2C, 2D. Shortly thereafter the first quartet of micromeres divide, also Iseotropically, forming two groups of four cells each known as la^ lb\ lc\ ld\ and la*, lb^ Ic*, ld^• the cells lying toward the animal pole are designated by the lower exponent.

The fifth cleavage, dividing the sixteen cells into thirty-two, in general resembles the preceding but is dexiotropic throughout. As the result we have the macromeres, 3A, 3B, 3C, 3D; a third quartet of micrpmeres, 3a, 3b, 3c, 3d; the second quartet of micromeres divides into 2aS 2bS 2c*, 2dS and 2a^, 2b^, 2c', 2d^ while the eight cells previously derived from the first quartet of micromeres now form la^S la", la'^ la", lb", lb", lb", lb", Ic", Ic", Ic", Ic", Id", Id", Id", Id",

After thirty-two cells have been formed in this fairly regular fashion, the rhythm of cleavage becomes modified so that there follow stages of 40, 44, 45, 53, 61 cells, etc

It is unnecessary for us to go farther with the details of these cleavages save in one particular. After the thirty-twocell stage the macromeres divide again unequally giving off a group of large cells which contain most of the deutoplasm of the original ovum. In spite of their size relations these large cells are known as the fourth quartet of micromeres, 4a, 4b, 4c, 4d, and the remaining smaller cells as the fourth quartet of macromeres, 4A, 4B, 4C, 4D. This contradiction in terminology is justified by the later history of these cells.

We may now consider the fates of these various groups of blastomeres. Quoting from Surface, "From the first quartet [of micromeres] arises the ectoderm, covering the anterior and dorsal portions of the body. From cells of this quartet four strings of cells bud into the interior of the embryo and form the ganglion. The eyes arise in ectodermal cells of this quartet. The second quartet gives rise to the larger portion of the ectoderm on the ventral and posterior regions of the body. \ From cells of this quartet is formed most of the ectodermal pharynx. A portion of the second quartet is budded into the embryo and forms mesoderm. From this source arises probably only that mesoderm formed around the blastopore and which is later concerned in the structures of the pharynx.

"The third quartet consists of small cells from which apparently only ectoderm is derived. The individual divisions of these cells have not been traced very far, but there is every reason to believe that they form ectoderm only.

Fig. 110. — Cleavage and cell lineage in the Poly dad Turbellarian, Planocera inguUina. From Surface. A. Egg during the first cleavage; side view. The cell C-D is slightly larger than A-B. B. Four-cell stage, from animal pole. C Formation of first quartet, from right side, showing spiral cleavage (dezio tropic). D. Eight-cell stage, from animal pole. E. Eight-cells dividing into sixteen, showing Isotropic division. The division of the cells of the Z> quadrant is in advance of the others. F. Sizteen-cells, from animal pole. O, Sixteen-cells, from vegetal pole.

Fig. 120. — Continuation tA Fig. 119. H. Deiiotropio diTiaon of la^liP and of 2a~2d. From animal pole. I. Thirty-two-cells, from animal pole. J. Thirty-two-cells from vegetal pole. K. Tliirty-two-oells from right side. L. Late cleavage showing the history of cells ia—4d and 4A-iD. M. Optical section of a much lat«r atage, viewed from near the vegetal pole. The mesodenn banda are stippled.

"The history of the fourth quartet is peculiar

The posterior cell 4d is the mesentoblast, from which the alimentary canal and a portion of the mesoderm arise. The other three cells of the fourth quartet, 4a, 4b, 4c, do not divide as long as their history can be traced. They, however, break up into a large number of homogeneous yolk spheres which are absorbed by the endoderm cells. The large nuclei of these three cells can be traced until the alimentary canal is partly formed.


Condensed from Surface.




A (left)

B (ant.)



^ D (post.)









lai I



























2b 1




























4a, 4A 4b, 4B 4c, 4C 4D

» apical cells.

]^ 1^113212

]^ 1^112218 ]^q112212 ]^ (^112212

primary ganglion cells.

2a 2b 2c 2d

» mesoblast.

yolk (no cell descendants).

' 4d' — ^alimentary canal (endoderm).

4d <






4d*" — probably endoderm.

4d* " — mesoderm(right "mesoblast band").

4d"^ — probably endoderm.

4^j222 — mesoderm (left

" mesoblast band").

The remaining cells form covering ectoderm. Ectoderm of first quartet — anterior and dorsal, including eyes. Ectoderm of second quartet — posterior and ventral, including pharyngeal. Mesoderm of second quartet — blastopofal (pharyngeal).

The nuclei of the small macromeres [4A, 4B, 4C, 4D] show evidences of degeneration. These do not divide as long as they can be followed and it seems probable that they degenerate without giving rise to any morphological structure."

This cell lineage of Planocera is summarized incompletely in the accompanying table.

It is interesting to compare with this lineage* of Planocera that of Ascaris, described by Zur Strassen, which is somewhat less regular. This is particularly interesting as it shows clearly the history of the germ cells, which become wholly separate from somatic cells in the sixteen-cell stage. Cleavage of Ascaris (Fig. 121) is bilateral but more or less irregular, particularly in its rhjdihms, so that without attempting to apply the ordinary terminology completely we may summarize the early cell history in the table accompanying (p. 255) .

The cell lineage of a considerable nimiber of organisms has been definitely traced, often in much greater detail than we have indicated. The histories best known are found among the Platyhelminthes, Nemathelminthes, Nemertinea, Annulata, Trochelminthes, MoUusca, and Tunicata. Of course the eggs of many classes and phyla show no such regularity, for as we have pointed out, cleavage may be irregular as well as indeterminate. And in many of the groups named above, normal development may occur even though interrupted by the removal of parts, by pressure, etc.

Fig. 121. — Cleavage in AtcarU megalticephalabij'alena. From Jenkinsoi). after Boveri. 1. Division of the twi>cell stage. EtimiDBtion of cbiomatin in the Bomatic cell Si(AB). la. ChromosomeB of the cell Si{AB}. 2. Four-cell BtBBS (T-form). In A and B can be seen the eliminated chromatin. The cell Pi has divided into a eomatic cell SilEMSl), in the deaoendants of which chromBtin elimination occurs, and the cell Pi. 3. Four-cell stage (loienge-form). A is anterior, A and B, dorsal. 4. Continued chromatin elimination in somatio cells. Pi has divided into Pi, and S.(C)— second ary ectoderm, a, b, primary ectoderm of right dde. •>, ff, of left side. 6. The endodcrm cell has been formed find has divided (Si, Et). Pi has divided into Pi, the primordial germ celt, and St(d), tertiary ectoderm, 6. Ventral view at the beginning of invagination, Elimination of chromBtin in iS-(J^). The four endoderm cells (£) beginning to invBginate. On each side two mesoderm cells (JIf) in which granular chromosomes may be seen, and two stomodttal cells (St).


Modified from Zur Strassen (Letters in parenthesis are the notation of Zur Strassen, Boveri and others)

9 O





r A




a <

a' («I)

AB '

^ (a)

a* (all)



■ b








[(?) '

c^ (rt.)



r c



c*(left) (/cot)



c^ (rt.) (EI)

L (E)






(c) d*


I (Pa)

(8.) ((c))


, (v)

(8.) (D)

[ (P.)

I (P.)


• 13 »33


.19 .31


mesoblast I (posterior), stomodseal cells (anterior), mesoblast I (posterior), stomodseal cells (anterior), (rt.)




k endoderm I.

ectoderm II,

mesoderm III.

mesoderm II.

(ant.) (post.)

d"» Geft) d^^^ (rt.) rf"» (left) d»" (rt.)




Primordial germ cell.

The study of cleavage from this point of view discloses the fact, of the utmost importance in development, that blastomeres may be individually and specifically recognizable as morphological and morphogenetic units. They bear much the same relation to the whole cell group that the organs and tissues bear to the embryo or later organism. As distinct morphological and physiological units they represent real differentiations at a very early stage of development, and may truly be said to form embryonic rudiments of structures appearing later in the form of germ layers or derivatives of these.

Fig. 122. — Diagrame illustrBting the value oF the quartets in three Uiimala. From WilsoQ, "Cell." Ectoplasm is uoBhadtd; meBoplasm is dotted; endoplasm ia vertically ruled. A. The Polyelad, Leptoplana, Bhowing mCBOplasm formation in second quartet. (Compare Planocera. Fig. 120, where Surface finds mesoplasm in cell 4d descendants.) B. The Gasteropod, Crepidula. C. Tbe Pelecypod, Unio.

Not only this but comparison of the cell lineages of different classes and phyla often brings out the fact that particular cells can be identified and compared in diverse groups of animals, making it possible to apply the idea of homology to blastomeres and groups of blastomeres in the early embryo, as well as to the organs and parts of the fully formed organism. Cells may be vestigial, rudimentary, and the like, in the same way that organs may be. The three or four successively formed quartets of micromeres or even an individual cell, for example that known as 4d, can be identified and homologized both in origin and in fate, in the phyla Platyhelminthes, Annulata, and MoUusca (Fig. 122). Wilson has written ("The Cell, etc.y page 416): "Thus we find that the cleavage of polyclades, annelids and gasteropods shows a really wonderful argeement in form, yet the individual cells differ markedly in prospective value. In all of these forms three quartets of micromeres are successively formed according to exactly the same remarkable law of alternation of the spirals; and, in all, the posterior cell of a fourth quartet lies at the hinder end of the embryo in precisely the same geometrical relation to the remainder of the embryo; yet in the gasteropods and annelids this cell gives rise to the mesoblast-bands and their products, in the polyclade to a part of the archenteron, while important differences also exist in the value of the other quartets." (It should be added that in the Polyclad, Planocera, the particular cell mentioned gives rise to mesoblast also.)

Such conditions also illustrate how the facts of embryology may have a certain value, often very great, as evidence upon phylogenetic problems.

Often these similarities of structure can be carried back into the pre-cleavage stage, and in the uncleaved zygote or ovum before fertilization, substances can be identified which later become contained within restricted groups of similar cells. So that cleavage is in part to be regarded as a process by which specific substances or regions of the egg become segregated in different regions of the embryo, where each continues its normal differentiation during later developmental stages and gives rise to specific tissues or organs. In other words the process of differentiation is not limited to the later stages of development following cleavage; it occurs during and even preceding cleavage. In Ascaris, for example, each of the two cells resulting from the first cleavage is specific; in other forms each of the four or eight cells has already become differentiated. In forms Uke the Ascidians (Conklin) true differentiation has commenced in the uncleaved ova and has been carried to a very pronounced degree. And it is not at all unUkely that these differentiations of the undivided egg may represent the most essential and most fundamental differentiations of the organsim. This aspect of cleavage cannot be discussed satisfactorily here without encroaching widely upon the subject of the next chapter and it is therefore left at this point.

It is to be noted, however, in conclusion that while cleavage may have an important chemical significance of general character, and a general physiological significance, yet the process is primarily a specific process of development and not mere cell multiplication. The process is closely related in all its details to the structure of the egg and also to the structure of the adult, since this too is similarly related to egg structure. Many of the details of cleavage do not occur according to physical laws based upon space and time relations of the parts, but departures from what we should expect on the basis of such laws may result, (a) from the historical factor of the relationships of organisms and the process of descent, (6) from the teleological factor, for cleavage has a prospective significance, looking forward, as well as a retrospective significance — cleavage has its promorphology as well as its morphology.

References to Literature

Balfour, F. M., (See ref. Ch. III.)

Beard, J., (See ref. Ch. III.)

BovBRi, T., Die Polaritat von Ovocyte, Ei und Larve des Strongylocenrtrolus lividus. Zool. Jahrb. 14. 1901.

Bresslau, E., Beitrage zur Entwicklungsgeschichte der Turbellarien. I. Die Entwicklung der Rhabdocolen und Alloicolen. Zeit. wiss. Zool. 76. 1901.

Conklin, E. G., (See ref. Ch. II, III.)

Driesch, H., Entwicklungsmechanische Studien. VIII. Ueber Variation der Mikromerenbildung. (Wirkung von Verddnnung des Meerwasser). Mitt. Stat. Neapel. 11. 1893.

GoDLEWSKi, E., Plasma und Kernsubstanz in der normalen und der durch gussere Factoren veranderten Entwicklung der Echiniden. Arch. Entw.-Mech. 26. 1908.

Habgitt,* C. W., Some Problems of Coelenterate Ontogeny. Jour. Morph. 22. 1911.

Hertwig, O., Das Problem der Befruchtung und der Isotropie dea Eies, eine Theorie der Vererbung. Jena. Zeit. 18 (11). 1885. KoRSCHELT UND HsroBR, Lehrbuch, etc. Ill Abschnitt. Furchung und Keimblatterbildung. Jena. 1909. LiLLiE, F. R., Adaptation in Cleavage. Woods Holl Biol. Lect. 1899.

Masing, E., (See ref. Ch. V.) MoENKHAUs, W. J., (See ref. Ch. II.) HoBERTy A., Recherches sur le developpement des Troques. Arch. Zool. Exp. (III). 10. 1903.

RucKERT, J., Ueber das Selbst&ndigbleiben der vaterlichen und mlitter lichen Kernsubstanz wahrend der ersten Entwicklung des befruch teten Cydops-Eies, Arch. mikr. Anat. 45. 1895.

Sachs, J., Ueber die Anordnung der Zellen in jlingsten Pflanzentheilen. Arbeiten Bot. Inst. Wtirzburg. 2. 1882.

Sblenka, E., Die Keimblatter der Echinodermen. Stud, uber Entw. II. Wiesbaden. 1883.

Surface, F. M., The Early Development of a Polyclad, Planocera inquilina, Wh. Proc. Acad. Nat. Sci. Philadelphia. 1907.

Watas^, S., Studies on Cephalopods. I. Cleavage of the Ovum. Jour. Morph. 4. 1891.

Wilson, E. B., The Cell-Lineage of Nereis. Jour. Morph. 6. 1892. Cell Lineage and Ancestral Reminiscence. Woods Holl Biol.

Lect. 1899. (See also ref. Ch. II.) Wilson, H. V., The Embryology of the Sea-Bass. (Serranua atrarius,)

BuU. U. S. Fish Com. 9. 1889. (1891). Zeleny, C, Experiments on the Localization of Developmental Factors

in'the Nemertine Egg. Jour. Exp. Zool. 1. 1904. Ziegler, H. E., Experimentelle Studien liber Zelltheilung. III. Die

Furchungszellen von Beroe ovata. Arch. Entw.-Mech. 7. 1898. ZuR Strassen, O., Embryonalentwickelung der Ascaris megalocephala.

Arch. Entw.-Mech. 3. 1896. Ueber die Lage der Centrosomen in ruhenden Zellen. Arch. Entw.-Mech. 12. 1901.

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Kellicott WE. A Textbook of General Embryology (1913) Henry Holt and Co., New York.

Kellicott (1913): 1 Ontogeny | 2 The cell and cell division | 3 The germ cells and their formation | 4 Maturation | 5 Fertilization | 6 Cleavage | 7 The germ cells and the processes of differentiation, heredity, and sex determination | 8 The blastxtla, gastrula, and germ layers. Morphogenetic processes

Cite this page: Hill, M.A. (2020, August 3) Embryology A textbook of general embryology (1913) 6. Retrieved from https://embryology.med.unsw.edu.au/embryology/index.php/A_textbook_of_general_embryology_(1913)_6

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© Dr Mark Hill 2020, UNSW Embryology ISBN: 978 0 7334 2609 4 - UNSW CRICOS Provider Code No. 00098G