Quain's elements of anatomy . ones, does not completely close themin, but leaves one or more of them visible by an aperture which has beencompared by Van Beneden, but according to some on insufficient groands,to the blastopore or aperture of invagination in the lower animals. Thisaperture is soon effaced by the union of the external cells over it, and inthis stage, which may be regarded as the completion of the segmentation,the ovum is covered externally by an entire layer of nucleated and some-what prismatic cells, while the interior is occupied by a solid mass ofcells of a different characte
Quain's elements of anatomy . ones, does not completely close themin, but leaves one or more of them visible by an aperture which has beencompared by Van Beneden, but according to some on insufficient groands,to the blastopore or aperture of invagination in the lower animals. Thisaperture is soon effaced by the union of the external cells over it, and inthis stage, which may be regarded as the completion of the segmentation,the ovum is covered externally by an entire layer of nucleated and some-what prismatic cells, while the interior is occupied by a solid mass ofcells of a different character. Thus the whole segmented ovum, whichis still only slightly increased in size, is converted into a hollow cellularsphere to which at a later period the name of hlastodermic vesicle isgiven. Partial Segmentation.—The process of segmentation as it occurs in meso-blastic ova contrasts widely in its more apparent phenomena with that pre-viously described, and yet, considered as one of protoplasmic cell-division, and Fig.
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