PHYLUM MOLLUSCA 



799 



extends inwards until it comes to underlie the whole of the 

 embryonic part of the blastoderm : separating this from the yolk 



}. (598. Sepia, blastoderm at a late stage of segmentation, bl. blastoderm ; yk. yolk. (From 

 Korschelt and Heider, after Vialleton.) 



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

 >9, yk. ep.}. There is some doubt as to the nature of the second 



ytt.ep 



. G99. Sections through the edge of the blastoderm of Sepia a 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 

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



3 E 2 



