. The physiology of the domestic animals; a text-book for veterinary and medical students and practitioners. Physiology, Comparative; Domestic animals. OEIGIN OF CELLS. 25 tinuing to surround the central hypoblastic spheres, both sets continuing to subdivide, until finally the ovum consists of an almost solid mass of hypoblastic spheres surrounded by a layer of epiblastic cells. When the process of segmentation is complete the epiblastic cells are clear and have an irregularly cubical form, while the hypoblastic cells are polygonal and granular and somewhat larger than the epi- blastic cells.
. The physiology of the domestic animals; a text-book for veterinary and medical students and practitioners. Physiology, Comparative; Domestic animals. OEIGIN OF CELLS. 25 tinuing to surround the central hypoblastic spheres, both sets continuing to subdivide, until finally the ovum consists of an almost solid mass of hypoblastic spheres surrounded by a layer of epiblastic cells. When the process of segmentation is complete the epiblastic cells are clear and have an irregularly cubical form, while the hypoblastic cells are polygonal and granular and somewhat larger than the epi- blastic cells. The blastodermic vesicle next forms. This results from the forma- tion of a narrow cavity between the epiblast and hypoblast, which increases in size until it entirely separates these two layers, except at the point where the blastoderm was last in forming (the blastopore). As the cavity increases in size the ovum also enlarges, so that soon it exists in the form of a large vesicle, formed of a thin wall of a single layer of. Fig. 15.—Optical Sections of a Rabbit's Ovum at Two Stages Closely Following Upon the Segmentation, after E. Van Beneden. (Balfour.) EP, epiblast; IIY, primary hypoblast; BP, Van Beneden's blastopore. The shading of the epiblast and hypoblast is diagrammatic. cells,—the epiblastic cells,—with a large cavity, the hypoblastic cells forming a small, ventricular mass attached to the inner side of the epi- blastic cells (Fig. 16). The ovum of the rabbit has now increased in size from mm., its size at the close of segmentation, to about mm. It is inclosed by the vitelline membrane and a mucous layer deposited by the walls of the Fallopian tube. As the vesicle continues to enlarge, the hypoblastic cells spread out beneath the epiblast, though remaining thicker in the centre than at the edges, where the cells still possess the power of amoeboid movement. The central, thicker portion, which is the commencement of the embryonic area, forms an opaque,
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