A. stage with eight segments still arranged in pairs, from above. B. side view of stage with eight segments. C. side view of stage with sixteen segments. D. side view of stage with forty-eight segments. E. view from above of stage with forty-eight segments. F. side view of embryo in the blastosphere stage, eight of the granular cells which give rise to the epiblast of the adult are present at the lower pole. _cs._ segmentation cavity; _ec._ granular cells which form the epiblast; _en._ clear cells which form the hypoblast.] The segmentation and early stages of development take place in the tissues of the parent. The segmentation is somewhat peculiar, though a modification of a regular segmentation. The ovum divides along a vertical plane, first into two, and then into four equal segments. But even when two segments are formed, each of them has one end pointed and the other broader. The pointed ends give rise to the ciliated cells of the future larva, and the broad ends to the granular cells. Instead of the next division taking place, as is usually the case, in a horizontal (equatorial) plane, it is actually effected along two vertical planes intermediate in position between the two first planes of segmentation. Eight equal segments are thus formed, each of which has the form of a pyramid. All the segments are situated in a single tier, and are so arranged as to give to the whole ovum the form of a flat cone, the apex of which is formed by the pointed extremities of the constituent segments (fig. 63 B). The apices of the segments do not however quite meet, but they leave a central space, which is an actual perforation (fig. 63 A) through the axis of the ovum, open at both ends. The first indications of this perforation appear when only four segments are present, and it is to be regarded as the homologue of the segmentation cavity of other ova. The next plane of division is horizontal (equatorial), and the apices of the eight cells are segmented off as a tier of small cells. At the completion of this division (fig. 63 C), the ovum is formed of sixteen cells arranged in two superimposed tiers. The ovum now assumes somewhat the form of a biconvex lens, in the axis of which the central perforation is still present. At the close of the next stage, forty-eight cells are present arranged in four tiers (fig. 63 D and E), the two outer tiers containing eight cells each, and the two inner sixteen. The two inner tiers probably arise by the simultaneous appearance of two equatorial furrows dividing the original tiers into two, and by the subsequent simple division of the cells of the two inner of the tiers so formed. At the close of the stage the eight basal cells become granular (fig. 63 F). At the same time the central part of the segmentation cavity becomes enlarged, while its terminal apertures become narrowed and finally, shortly after the end of this stage, closed. The axial