732 



ZOOLOGY 



SECT. 



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 

 prototroch or future velum is formed. Subsequently the position 

 of the blastopore becomes still further shifted and its form U-shaped 

 and then slit-like. It undergoes elongation (Fig. 655, A) and 



B 



shgl 



FIG. 654. Earlier stages in the development of Patella. A, blastula ; B, beginning of endo- 

 dermal inyagination ; C, completion of gastrula ; D, frontal section of somewhat later stage. 

 ap. pi. apical plate ; bl. blastopore ; endm. endo-mesoderm cell ; end. endoderm ; mes. meso- 

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

 Heider, after Patten.) 



eventually becomes partly closed up, the closure taking place 

 from behind forwards ; in most cases in the position of the anterior 

 part a sinking-in of the ectoderm forms the rudiment of the 

 stomodseum. The originally solid mass of endoderm develops a 

 lumen, and its cells become arranged to form the enteric epithelium. 

 From the posterior end, where the mesoderm cells are situated, 

 proceed two very regularly formed mesoderm-bands (Fig. 655, B). 



