. The physiology of domestic animals ... Physiology, Comparative; Veterinary physiology. 24 PHYSIOLOGY OF THE DOMESTIC ANIMALS. hypoblast (Fig. 14). From these three layers of cells the embryo is developed. Leaving at this point the changes which occur in the egg of the bird, we have now to follow the analogous changes in the mammalian ovum. We have already seen that in the mammalian ovum one of the first evidences of impregnation is the division of the protoplasm of the ovum progressively into smaller and smaller segmentation spheres, until the cell-membrane becomes filled with an immense num
. The physiology of domestic animals ... Physiology, Comparative; Veterinary physiology. 24 PHYSIOLOGY OF THE DOMESTIC ANIMALS. hypoblast (Fig. 14). From these three layers of cells the embryo is developed. Leaving at this point the changes which occur in the egg of the bird, we have now to follow the analogous changes in the mammalian ovum. We have already seen that in the mammalian ovum one of the first evidences of impregnation is the division of the protoplasm of the ovum progressively into smaller and smaller segmentation spheres, until the cell-membrane becomes filled with an immense number of minute masses of protoplasm. The general character of this process in its earlier stages is probably identical in all the mammalia. The ovum of the rabbit has been most studied, and the sketch here given is based mainly on Balfour's summary of the early stages of development in the rabbit's ovum. The ovum first divides into two nearly equal spheres, of which one. Fig. 14.—Section of a Blastoderm of Chick, at Right Angles to the Long Axis of the Embryo, after Eight Hours' Incubation, about Midway Between Front and Hind Ends. (Foster and Balfour.) A, epiblast; B, mesoblast; C, hypoblast; PR, primitive groove; F, fold in the blastoderm produced accidentally ; MC, uiesoblast-cell,—the line points to one of the peripheral inesoblast-cells lying between epiblast and hypoblast; BD, formative cells. The section shows: (1) the thickening of the mesoblast under the primitive groove, PR, even when it is hardly present at the sides of the groove; (2) the hypoblast, C, earlyformed asa single layer of spindle- shaped cells; (3) the so-called segmentation cavity, in which coagulated albumen is present. On the floor of this are the large formative cells, BD. is slightly larger and more transparent than the other. The larger sphere and its products will be spoken of as the epiblastic spheres; the smaller one and its products as the hypoblastic spheres. Both these original spheres soo
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