Quain's elements of anatomy . the two sides approach oneanother so as gradually to narrow and at last obliterate the original cavity whichlay between them. The part which remains outside forms the external layer orectoderm, and that which is doubled in is the internal layer or entoderm. Theprotembryo or blastoderm thus assumes the form of the bilaminar gatrula ofHaeckel, communicating with the exterior by the now narrowed aperture calledblastopore, and representing in fact the simplest form of an alimentary cavity. Complete Segmentation in Mammals.—The Mammals comenext to the Amphioxus in the
Quain's elements of anatomy . the two sides approach oneanother so as gradually to narrow and at last obliterate the original cavity whichlay between them. The part which remains outside forms the external layer orectoderm, and that which is doubled in is the internal layer or entoderm. Theprotembryo or blastoderm thus assumes the form of the bilaminar gatrula ofHaeckel, communicating with the exterior by the now narrowed aperture calledblastopore, and representing in fact the simplest form of an alimentary cavity. Complete Segmentation in Mammals.—The Mammals comenext to the Amphioxus in the completeness and regularity of the seg-mentation, but they difiPer from it both in the early distinction of theupper and lower blastodermic cells and in the absence of any obviousinvagination of the vesicular blastoderm. The segmentation of the mammiferous ovum had been seen by MartinBarry, but its general features were first clearly demonstrated by the im-portant researches of Bischoff. In more recent times much light has. Fig. 630.—First stages of se&mentatioh of a mammalian ovum : semi-diagrammatio.(Drawn by A. T. after Edwd. v. Benedens description.) s.^j, zona pellucida; , polar globules ; ect, ectomere ; ent, entoniere ; a, divisioninto two blastomeres ; b, stage of four blastomeres ; c, eight blastomei-es, the ectomerespartially enclosing the entomeres ; , succeeding stages of segmentation shoAvingthe morerapid division of the ectomeres and the enclosure of the entomeres by them. been thrown upon the nature of this process by the interesting researchesof Edwd. Yan Beneden (ISTo. 74) and others. From the observationsof Yan Beneden it appears that in the ovum of the rabbit within one ortwo hours after the union of the male and female pronuclei the processof division of the nucleus and the primary segment sphere commences. 744 THE FECrNDATED OVUM. This, as well as all the subsequent divisions which occur, is accompaniedby the usual phenomena of spindle modificat
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