800 



ZOOLOGY 



SI!fT. 



mesenteron. From whatever source it may be derived, the latter 

 becomes distinguishable as a cell-plate which is converted 

 into a vesicle opening below against the yolk-epithelium, there 

 never being any direct communication with the yolk. An exten- 

 sive stomodamm eventually opens into this ; a proctodaBum is 

 merely represented by the ectodermal pit forming the anus. 



FIG. 700. Early stages in the development of Loligo A, stage at which the rudiments of the eyes 

 and of the shell-gland are first distinguishable ; B, later embryo from the oral side ; C and D, 

 from the anal side. ant. f. f. anterior funnel-fold; ar. rudiments of arms; cten. ctenidia ; 

 eye, eye ; mo. mouth ; mant. rudiment of mantle ; ot. statocyst ; post. /. /. posterior funnel-fold ; 

 sh. gl. shell-gland ; yk. s. yolk-sac. (After Korschelt and Heider.) 



About the middle of the blastoderm appears a thickening 

 of a cap-like shape, the edges of which become raised above the 

 general level of the blastoderm ; this is the rudiment of the 

 mantle. On the surface of this is developed a depression which 

 subsequently forms a closed sac the shell- gland (Fig. 700, sh.gl). 

 Below the mantle i.e. nearer the vegetal pole appear two eleva- 

 tions each with a pit-like depression, which are the rudiments of 



