356 SEGMENTATION. 



cells is eventually formed, which completely envelops the four 

 large spheres except for a small blastopore at the vegetative pole 

 of the ovum (fig. 160 A). The large spheres continue to give 

 rise to smaller cells which however no longer take a superficial 

 position but lie within the layer of small cells, and give rise to 

 the hypoblast (fig. 160 B). The small cells become the epiblast, 

 and at the blastopore they curl inwards (fig. 160 B) and give 



FIG. 160. EPIBOLIC GASTRULA OF BONELLIA. (After Spengel.) 



A. Stage when the four hypoblast cells are nearly enclosed. 



B. Stage after the formation of the mesoblast has commenced by an infolding of 

 the lips of the blastopore. 



ep. epiblast ; me. mesoblast ; bl. blastopore. 



rise to a layer of cells, which appears to extend as an unbroken 

 sheet between the epiblast and hypoblast, and to form the 

 mesoblast. The blastopore now closes up, but its position in 

 relation to the parts of the embryo has not been made out. 



In Phascolosoma (Selenka, No. 369) the ovum, enclosed in a 

 porous zona radiata, divides into two unequal spheres, of which 

 the smaller next divides into two and then into four. An 

 invagination takes place which is intermediate between the 

 embolic and the epibolic types. The small cells, the number of 

 which is increased by additions from the large sphere, divide, and 

 grow round the large sphere. The latter in the meantime also 

 divides, and the cells produced from it form on the one hand a 

 small sack which opens by the blastopore, and on the other they 

 fill up the segmentation cavity, and become the mesoblast and 

 blood corpuscles. The blastopore becomes the permanent 

 mouth. 



