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External Features of Segmentation.
Fig. 38. Various stages in process of segmentation.
(After Gegenbaur.)
In the simplest known type of segmentation the ovum first of all divides into two, then four, eight, sixteen, thirty-two, sixty-four, etc. cells ([fig. 38]). These cells so long as they are fairly large are usually known as segments or spheres. At the close of such a simple segmentation the ovum becomes converted into a sphere composed of segments of a uniform size. These segments usually form a wall ([fig. 39], E), one row of cells thick, round a central cavity, which is known as the segmentation cavity or cavity of Von Baer. Such a sphere is known as a blastosphere. The central cavity usually appears very early in the segmentation, in many cases when only four segments are present ([fig. 39], B).
Fig. 39. The Segmentation of Amphioxux.
(Copied from Kowalevsky.)