IX 



MOLLUSCA 



359 



mes 



FIG. 290. Two sagittal sections of early embryos 

 of Loliyo vulgar is in order to illustrate the 

 first formation of organs. (After Faussek.) 



A, Staj;e in which the endoderm is first visible as a 

 differentiation of the mesoderm. B, Stage in which the 

 ink sac is formed, end, endoderm ; i.s, ink sac ; w \ 

 mesoderm s..v, shell-sac ; stem, stomodaeum ; y.m, yolk- 

 membrane. 



