PHYLUM MOLLlSt A 



:<.»<) 



extends inwards until it comes t<» underlie the whole of the 

 embryonic part of the blastoderm : separating this from the yolk 



hi 



Fiu. til's.— Sepia, blastoderm at a late stage of segmentation, hf. blastoderm ; yk. yolk. (From 

 Korschelt and Heider, after Vialleton.) 



is a thin layer of uncertain derivation — the yolk-cpithclivm (Fig. 

 699, yk. itp..). There is some doubt as to the nature of the second 



Fio. 099.— Sections through the edge of the blastoderm of Septa at three successive stages ; 

 W. blastoderm ; yk. yolk ; yk. ep. yolk-epithelium. (From Korschelt and Heider, after 

 Vialleton.) 



layer ; it certainly gives rise to the mesodermal structures, and 

 by some observers it is also said to form the epithelium of the 



3 E 2 



