800 



ZOOLOGY 



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 stomodseum eventually opens into this ; a proctodeum 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 ; and D, 

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

 eye, eye ; mo. mouth ; want, rudiment of mantle ; ot. statocyst ; post. f. f. 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 



