:;(!•_' 



CEPHALOPODA. 



The cerebral ganglion arises in the form of an ectodermal thicken- 

 ing above the rudiment of the stomodaeum. Before the latter sinks 

 in, the ectoderm above this region becomes mnltilaminar (Fig. 131 



IC 



t 



— eet. 



«*.- 



Fig. 146. — Sagittal sections through embryos of Loligo of various ages, somewhat 

 diagrammatic (original). Section B is made in the region of the mouth, a, anal 

 ion; "/'. arm-rudiment ; c, cerebral commissure; eg, cerebral ganglion: if, yolk; 

 </<•, yolk-epithelium ; ect, ectoderm ; m, mouth ; ma, mantle-fold ; md, enteron ; mes, 

 mesoderm (diagrammatic); r, radularsac; sd, shell-gland; sp, salivary-gland; tb, 

 ink-sac ; od, stomodaeum. 



D, ect., p. 280). This ectodermal layer, which at first is thin, 

 thickens greatly after the stomodaeum has become invaginated (Fig. 

 1 Hi A, eg), and a large ganglionic mass forms at this point, consisting 



