. Quain's elements of anatomy . e two sides approach oneanother so as gradually to narrow and at last obliterate the original cavity whichlay between them. The part which remains outside f omis the external layer orectoderm, and that which is doubled in is the internal layer or entoderm. TheIDrotembryo or blastoderm thus assumes the foim of the bilaminar gastnda 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 th
. Quain's elements of anatomy . e two sides approach oneanother so as gradually to narrow and at last obliterate the original cavity whichlay between them. The part which remains outside f omis the external layer orectoderm, and that which is doubled in is the internal layer or entoderm. TheIDrotembryo or blastoderm thus assumes the foim of the bilaminar gastnda 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 differ 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 Avere first clearly demonstrated by the im-portant researches of BischoflF. In more recent times much light has. Pig. 630.—FlKST STAGES OF SEGMENTATION OF A MAJniALIAN OVUM : SEMI-DIAGRAMMATIC. (Drawn by A. T. after Edwd. v. Benedens description.) z.}), zona pellucida; , polar globules ; ed, ectomere ; ent, entomere ; a, divisioninto two blastomeres ; b, stage of four blastomeres ; c, eight blastomeres, the eetomerespartially enclosing the entomeres ; , succeeding stages of segmentation showing the morerapid division of the eetomeres and the enclosure of the entomeies by them. been thrown upon the nature of this process by the interesting researchesof Edwd. Van Beneden (No. 74) and others. From the observationsof Van Beneden it appears that in the ovum of the rabbit within one ortwo homS after the union of the male and female pronuclei the processof division of the nucleus and the primary segment sphere commences. 744 THE FECUNDATED OYUM. This, as Avell as all the subsequent divisions which occur, is accompaniedby the usual phenomena of spindle modificati
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