File:The cat - an introduction to the study of backboned animals, especially mammals (1881) (19964191124).jpg

Title: The cat : an introduction to the study of backboned animals, especially mammals
Identifier: catintroduction00miva (find matches)
Year: 1881 (1880s)
Authors: Mivart, St. George Jackson, 1827-1900
Subjects: Cats; Anatomy, Comparative
Publisher: New York : Scribner's
Contributing Library: The Library of Congress
Digitizing Sponsor: The Library of Congress

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CHAP. X.) TEE DEVELOPMENT OF THE CAT. 319 nuclei, and the protoplasm of the ovum also divides, arranging itself around each secondary segmentation nucleus. The two halves are not quite equal in size. One, larger and more transparent (Fig. 141, B, e) is the ectodermic or epiblastic sphere, the other (i) is the endodermic or hypoblastic sphere. Each sphere again subdivides, and the same process is repeated with each subdivision, there being thus four spheres, or cells, derived from the primitive epiblastic sphere, and four from the primitive hypoblastic sphere. One of these latter now assumes a central position (Fig. 141, C). The process of division is then continued, but unequally, the cells derived from the epiblastic sphere dividing more rapidly, and therefore be- coming smaller than these from the hypoblastic sphere. Moreover
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Impregnated ovum, with a single segmenta- tion nucleus. h. External envelope. z. Zona pellucida, with spermatozoa. n. Segmentation nucleus. y. Protoplasm or yelk of the ovum. Fig. 141.—Yelk*Segmentation. B. Ovum, with the yelk divided into two cleavage cells, each with its nucleus. e. Epiblastic cleavage cell, i. Hypoblastic cleavage cell. C. Fourth segmention into eight cells—four epiblastic and four hypoblastic. the epiblastic cells come to surround and enclose the hypoblastic cells, save, perhaps, at one point where the hypoblastic cells may appear at the surface (Fig. 142). The mass of cells, however, forms a solid whole. There is no central cavity. Soon, however, a clear space appears, and liquid forms between the mass of hypoblastic cells and the epiblastic cells (Fig. 143, A), save at one part where the two sets of cells adhere together. This process rapidly continues as the ovum grows, the interspace becoming' wider and wider, till at last we have a relatively large sphere of very small epiblastic cells, against the inside of one part of which the aggregation of small hypoblastic cells is flattened out, so that we have a double stratum of cells, consist- ing of a single layer of hypoblastic cells beneath a single layer of epi- blastic cells (Fig. 143, B). In other words, we have two cellular membranes—an epiblast and a hypoblast—the epiblast investing the whole ovum within the vitelline membrane, the hypoblast under- lying only a portion, though a considerable portion, of the epiblast. The cellular membrane thus investing the ovum, and derived from the segmentation of the yelk, is called the blastoderm, and the whole structure is the blastodermic vesicle. That part where the two

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