302 



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 multilaminar (Kir. 1 :'> 1 



FIG. 146. Sagittal sections through embryos of Lol-igo of various ages, somewhat 

 diagrammatic (original). Section /; is made in the region of the moutli. <>, anal 

 region; ar, arm-rudiment; c, cerebral commissure; eg, cerebral ganglion; <l, yolk ; 

 de, yolk-epithelium ; ect, ectoderm ; m, mouth ; ma, mantle-fold ; md, enteron ; HK-H, 

 inesoderm (diagrammatic); r, radular sac ; sd, shell-gland; xj>, salivary-gland; //', 

 ink-sac ; vd, stomodaeum. 



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

 thickens greatly after the stomodaeum has become invaginated (Fig. 

 146 A, eg), and'a large ganglionic mass forms at this point, consisting 



