Fig. 63. Successive stages in the segmentation of Sycandra raphanus.
(Copied from F. E. Schulze.)
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 perforation thus acquires the character of a closed segmentation cavity. While the ovum itself becomes at the same time a blastosphere.
Fig. 64. Larva of Sycandra raphanus at pseudogastrula stage, in situ in the maternal tissues. (Copied from F. E. Schulze.)
me. mesoblast of adult; hy. collared cells forming hypoblast of the adult; en. clear cells of larva which eventually become involuted to form the hypoblast; ec. granular cells of larva which give rise to the epiblast, which at this stage are partially involuted.
This stage nearly completes the segmentation: in the next one, the cells of the poles of the blastosphere increase in number, and the cells of the greater part of the blastosphere become columnar and ciliated, ([fig. 64] en.) while the granular cells (ec.) increase to about thirty-two in number and appear to be (partially at least) involuted into the segmentation cavity, reducing this latter to a mere slit. This stage forms the last passed by the embryo in the tissues of the parent. The general position of the embryo while still in this situation may be gathered from [fig. 64], representing the embryo in situ. The embryo is always placed close to one of the radial canals. From this situation it makes its way through the lining cells into a canal and is thence transported to the surrounding water. By the time the larva has become free, the semi-invaginated granular cells have increased in bulk and become everted so as to project very much more prominently than in the encapsuled state. To the gastrula stage, if it deserves the name, passed through by the embryo in the tissues of the parent, no importance can be attached.