XII 



PHYLUM MOLLUSCA 



745 



dors 



it consists of a single layer of cells the ectoderm. Later on a 

 thickening is observable at the periphery, due to the development 

 of a second layer, which by 

 degrees extends inwards until 

 it comes to underlie the entire 

 ectoderm. This second layer 

 is commonly termed the meso- 

 derm, though it differs from 

 the middle layer of other em- 

 bryos in giving rise to the 

 enteric epithelium of the 

 mesenteron. The ectoderm 

 now extends rapidly, and even- 

 tually encloses the entire yolk. 

 Below the second layer extends 

 a thin stratum derived from 

 the peripheral protoplasm, con- 



posl 



vent 



FIG. 656. Segmenting ovum of LoligO. 

 (From Korschelt and Heider, after Watase.) 



taining nuclei which appear to 



have been previously scattered 



through the yolk ; this is 



termed the yolk epithelium (Fig. 658). It serves the purpose of 



absorbing the yolk-substance for the benefit of the developing 



/ * v^y '' v'' ; *v*'-*'/*\' r 



* Jr\*^(ii >I ;*-..>' *>_^ 



j/>fe 



p lc 657 Sepia blastoderm at a late stage of- segmentation. W. blastoderm ; yk. yolk. (From 



Korschelt and Heider, after Vialleton.) 



embryo. About the middle of the blastoderm appears a thickening 

 of a cap-like shape, the edges of which become raised above the 



