PHYLUM MOLLUSCA 



701 



is most typically developed in Patella ; in other Gastropods it 

 undergoes more or less modification. In Patella (Fig. 607) there 

 is a ciliated blast ula (A) which has on one side the large megameres. 

 The latter become enclosed by the micromeres, and the foundation 

 of the mesoderm is laid in the manner already described. The 



B 



FIG. 607. Earlier stages in the development of Patella. A, blastula ; B, beginning of endo- 

 dermal invagination ; C, completion of gastrula ; Z>, frontal section of somewhat later stage ; 

 ap. pi. apical plate ; bl. blastopore ; endm. endomesoderm cell ; end, endoderm ; mes. meso- 

 derm ; mesent. mesenteron ; pro to. prototroch ; sh. gl. shell-gland. (From Korschelt and 

 Heider, after Patten.) 



blastopore is situated at the vegetal pole, destined to become 

 the hinder end of the larva. The blastopore soon changes its 

 position and extends forwards on the ventral side, and a ciliated 

 ring the prototroch or future velum -becomes formed. Subse- 

 quently the position of the blastopore becomes still further 

 shifted; it becomes U-shaped and then slit-like. It undergoes 



