PORIFERA. 



139 



intermediate in position between the two first planes of segmen- 

 tation. 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, 





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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 blast osphere stage, eight of the granular cells which 

 give rise to the epiblast of the adult are present at the lower pole. 



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

 which form the hypoblast. 



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 superim- 

 posed tiers. The ovum now assumes somewhat the form of a 

 biconvex lens, in the axis of which the central perforation is still 



