752 



ZOOLOGY 



SJKCT. 



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

 blastopore is situated at the vegetal pole, destined to become 

 the hinder end of the larva, but it soon changes its position 

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

 — the 2^'>''ototroeh or future velum — is formed. Subsequently 



Fig. 644. — Earlier stages in the development of Fatella. A, blastula ; E, beginning of ondo- 

 dernial invagination ; C, cotnpletion of gastrula; D, frontal section of somcwliat later stage. 

 ap. pf. apical plate ; bl. blastopore; endia. endo-raesoderni cell; ead. endoderm ; mefi. meso- 

 derm; ui>'.^t;>}l. mesenteron ; yro^o. prototroch ; sh. [ft. shell-gland. (From Korschelt and 

 Heider, after Fatten.) 



the position of the blastopore becomes still further shifted and 

 its form U-shaped and then slit-like. It undergoes elongation 

 (Fig. 645, A) and eventually becomes partly closed up, the 

 closure taking place from behind forwards ; the most anterior 

 part remains open to form the mouth — or, perhaps more correctly, 

 there is in the position of the anterior part a sinking-in of the 



