IX 



MOLLUSCA 



359 



ss 



sio/n i 



Fig. 290. — Two sagittal sections of early embryos 

 of Lollgo vulgaris in order to illustrate the 

 first formation of organs. (After Fausselt.] 



A, Stage in "which the endoderm is finst visible as a 

 differentiation of tlie mesoderm. B, Stage in wliich the 

 inlt sac is formed, end, endoderm ; i.s, ink sac ; mes, 

 mesoderm s.s, sliell-sac ; s^om, stomodaeum ; t/.tti, yolk- 

 laembrane. 



