XII 



PHYLUM MOLLUSCA 



799 



extends inwards until it comes to underlie the whole of the 

 embryonic part of the blastoderm : separating this from the yolk 



Fin. r>OS. Sepia, blastoderm at a late stage of segmentation, bl. blastoderm ; ///.-. yolk. (From 



Korschelt and Heider, after Vialleton.) 



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

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



yk.ep 



FIG. G99. Sections through the edge of the blastoderm uf Sepia at three successive stages; 

 bl. blastoderm; yk. yolk; yk. cp. yolk-epithelium. (From Korsi-hult 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 



