Cleavage (embryology)

“ It has been experimentally proved that external stimuli (light, heat, pressure, &c.) are not responsible for the first differentiation of organs in the embryo; thus, should the segmented egg be absolutely equal in itself, it would be incomprehensible that the first organs should be formed at one special point of it and not at another. Besides this general argument, we see a sort of orientation in the typical forms of the polar or bilateral cleavage stages. ”

“ The end of cleavage is marked by the commencement of the differentiation of the organs. The first differentiation is the formation of the layers. These are three in number, being called respectively the ectoderm, endoderm and mesoderm, or, in embryos in which at their first The layer theory. appearance they lie like sheets one above the other, the epiblast, hypoblast and mesoblast. The layers are sometimes spoken of as the primary organs, and their importance lies in the fact that they are supposed to be generally homologous throughout the series of the Metazoa. ”

“ The fact that the regional differentiation may be foreshadowed in the egg before cleavage begins,56—that as Driesch has shown, the mode of cleavage may be artificially altered without affecting the ultimate organization of the embryo,—and many other similar observations, tend to emphasize the importance of the “organism” standpoint (C. O. Whitman, 1903, p. 642) in contradistinction to the widely prevalent “cell” standpoint. ”

“ If the ovum at this stage be looked at from above, the embryonic area appears as a small shaded oval. The shading is due to an increased growth of cells, because here the three germinal layers—embryonic ectoderm, mesoderm, and entoderm—are in contact. At one end a patch of darker shading indicates a still greater growth of cells. Running forward from this is a band—the primitive streak—in the centre of which lies a darker line—the primitive groove. ”

“ The cells which become ova increase in size, are passive, and rich in reserve material elaborated by their protoplasm. The cells which become sperms divide again and again, [43] and thus produce minute active bodies, adance with restless motion. These opposite tendencies are repeated and emphasized throughout the animal kingdom—ova relatively large, passive, and accumulative of reserve material; sperms minute, active, and the result of repeated fission. The active sperm, when it unites with the ovum, imports into it a tendency to fission, or cleavage ”

“ This is the first attempt of the fertilised ovum to form itself into the different layers, which are ultimately going to give rise to all the different tissues of the embryo. But it is interesting to know at this stage that the outer layer of cells, those representing a margin in the figure, has nothing to do with the forming of the embryo at all, but gives rise to a structure whose function afterwards is to be that of nourishing the growing embryo. ”

“ This process of fusion between the two kinds of reproductive cells, which are termed gametes, is called conjugation: it is the process which is sometimes spoken of as the fertilization of the ovum, and its result is the establishment of a new individual. This new individual at first is simply a uninucleated mass of living matter, which always contains a certain amount of food-yolk, and is generally bounded by a delicate cuticular membrane called the vitelline membrane. ”

“ The outlines of the embryo are far bolder than during the earlier stages. Fig. 109 shews the nature of the folding, by which the embryo is constricted off from the yolk-sack. The folds are complicated by the fact that the mesoblast has already become split into two layers—a splanchnic layer adjoining the hypoblast and a somatic layer adjoining the epiblast—and that the body cavity between these two layers has already become pretty wide in the lateral parts of the body of the embryo and the area pellucida. ”

“ Cleavage of the Egg.—It is through a series of such divisions as these that the zygote or fertilized egg-cell builds up the tissues and organs of the new organism. The process is technically spoken of as cleavage. Cleavage generally begins very shortly after fertilization. The fertile egg-cell divides into two, the resulting cells divide again and thus the process continues, with an ever-increasing number of cells. ”

“ For instance, in the wall of the medullary canal in the Vertebrate embryo, in the splanchnic and somatic layers of mesoderm of the same embryo, and in the developing nerve cords of the Peripatus embryo, the nuclei are at first so densely crowded together that it is almost impossible to see the protoplasmic framework in which they rest, but as development proceeds this extra-nuclear tissue becomes more largely developed, and the nuclei are forced apart, so that it becomes visible and receives various names according to its position. ”

“ The growth of this aster, apparently by linear rangings of the granules composing the reticulum of the germ-cell, progresses rapidly; while the whole structure hence arising moves inward. Soon there takes place the fusion of this sperm-nucleus with the germ-nucleus to form the cleavage-nucleus, which, after a pause, begins to divide and subdivide in the same manner as cells at large: so presently forming a cluster of cells out of which arise the layers originating the embyro. ”

“ The position of the primitive streak immediately behind the embryo suggests the speculation that it may represent the line along which the edges of the blastoderm coalesced, so as to give to the embryo the central position which it has in the blastoderms of Birds and Mammals, and that the peculiar fusion of epiblast and mesoblast at this point may represent the primitive continuity of epiblast and lower layer cells at the dorsal lip of the anus of Rusconi in Elasmobranchii. ”

“ Each successive step from the very beginning of the fertilisation of the ovum repeats a stage through which previous generations have passed. If from any accident a step in this recapitulation is omitted, the embryo is to that extent deprived of some feature possessed by a parent or ancestor; and if this be a sufficiently important omission, it is impossible for such an embryo to survive. That is one way in which an embryo may differ from its parents. ”

“ As the human embryo does not essentially differ, even at a much later stage of development—when we already perceive the cerebral vesicles, the eyes, ears, gill-arches, etc.—from the similar forms of the other higher mammals, we may confidently assume that they agree in the earliest embryonic processes, segmentation and the formation of germinal layers. This has not yet, it is true, been established by observation. We have never yet had occasion to dissect a woman immediately after impregnation and examine the stem-cell or the segmentation-cells in her oviduct. ”

“ Germinal Layers and Food-Canal.—The embryo begins to develop as an elongate, thickened, ventral region of the blastoderm which is known as the ventral plate or germ band. Along this band a median furrow appears, and a mass of cells sinks within, the one-layered germ band thus becoming transformed into a band of two cell-layers (fig. 15) . In some cases the inner layer is formed not by invagination but by proliferation or by delamination. The outer of these two layers (fig. 15, E) is the ectoderm. ”

“ In its formation this involution pushes backwards the segmentation cavity, which finally disappears in the stage C III. The point x remains practically stationary, but by the general growth of the epiblast, mesoblast and hypoblast, becomes further removed from the segmentation cavity in C II than in C I. On the opposite side of the embryo to that at which the involution occurs the epiblast cells as before, grow round the lower layer cells. ”

“ Embryology has shown us that, during gestation, each human embryo is a replica of the past; it passes through the different Imago stages from protoplasm to man, being unrecognisable at certain stages from a monad, an amœba, a fish with gills, a lizard, and a monkey with a tail and dense clothing of hair over the whole body. The human embryo has also, at an early stage, the thirteenth pair of ribs, which is found in lower animals and is still seen in a rudimentary form in anthropoid apes, but which disappears from the human embryo before birth. ”

“ The embryonic shield is mainly distinguished from the remainder of the blastoderm by the more columnar character of its constituent epiblast cells. It is somewhat pyriform in shape, the narrower end corresponding with the future posterior end of the embryo. At the hind end of the shield a somewhat triangular primitive streak is formed, consisting of epiblast continuous below with a great mass of rounded mesoblast cells, probably mainly formed, as in the bird, by a proliferation of the epiblast. To this mass of cells the hypoblast is also partially adherent. ”

“ Indeed the segregation and isolation of different protoplasmic substances in different cleavage cells occurs during the cleavage of the egg in all animals, though such differentiations are much more marked in some cases than in others.
Fig. 31. Gastrula and Larva of Styela, showing the cell lineage of various organs, and the distribution of the different kinds of protoplasm to these organs. Muscle cells are shaded by vertical lines, mesenchyme by horizontal lines, nervous system and chorda by stiples. ”

“ The internal cavity is laid open so as to exhibit an embryo in each half of the Ovum, the one being double the size of the other. The smaller embryo is about as large as a good-sized house-fly. The cord of the largest embryo is nearly an inch and a half long, sacculent, and expanding towards the abdomen so as to form the only covering of that cavity. The cord of the smaller embryo is half an inch in length and filiform. ”

“ As soon as a spermatozoon enters an ovum, it disappears and is completely absorbed, and, as the ovum in turn is instantly possessed of new powers, the result of this union is a cell which was previously non-existent.
This new cell is not only capable of reproduction by means of segmentation or cell division, but in the course of its sub-division and proliferation, it forms groups of cells which develop into tissues and structures widely different from each other. The entire complex human body, in addition to the placenta, cord, and membranes, arises from the single, extraordinary cell. ”