114 



SYCANDRA. 



central space, which is an actual perforation (fig. G3 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 



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. 

 G. 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. 



c s. segmentation cavity; ec. granular cells which form the epiblast; en. clear cells 

 which form the hypoblast. 



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 aper- 

 tures 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. 



