XII 



PHYLUM MOLHT.SCA 



799 



extends inwards until it comes to underlie the whole of the 

 embryonic part of the bbist(xlerm : separating this from the yolk 



Fic. 098. — Sepia, blastoderm :it a late stage of segmentation, bl. blastoderm ; ylc. yilk. (From 

 Korschelt and Heider, after Vialleton.) 



is a thin layer of uncertain derivation — the i/olk-ciiitJicli/rrn, (Fig. 

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



Fig. 099.— Sections through the edge of the blastoderm of Sepia at three successive stages ; 

 bl, 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 



