MOLLUSCA. 



263 



Lankester (No. 239). The ovum is invested by a vitelline membrane and 

 undergoes development in a brood-pouch at the base of the inner gill lamella. 



The segmentation commences by a division into four equal spheres, each 

 of which, as in so many other Mollusca, then gives rise by budcling to a 

 small sphere. The later stages of segmentation have not been followed in 

 detail, but the result of segmentation is a blastosphere. An invagination, 

 presumably at the lower pole, now takes place, and gives rise to an 

 archenteric sack. 



The embryo now rapidly grows in size. The blastopore becomes closed 

 and the archenteric sack forms a small mass attached at one point to the 

 walls of the embryonic vesicle (fig. 119, hy). In the space between the walls 

 of the archenteron and those of the embryonic vesicle stellate mesoblast cells 



B. 



FIG. 119. THREE VIEWS OF AN EMBRYO OF PISIDIUM IMMEDIATELY AFTER 

 THE CLOSURE OF THE BLASTOPORE. (After Lankester.) 



A. View from the surface. 



B. Optical section through the median plane. 



C. Optical section through a plane a little below 'the surface. 



ep. epiblast ; me. mesoblast ; hy. hypoblast ; /. cells apparently budding from the 

 hypoblast to form mesoblastic elements. 



