484 



PROCEEDINGS OF THE ACADEMY OF 



[Sept., 



-appears and subsequently divides to form the cerebro-pedal and 

 cerebro-pleural connectives. During this time the cells of the 

 anterior margin of each cerebral ganglion likewise elongate and 

 rapidly form fibres which extend across the midline and thus form 

 the cerebral commissure. 



The buccal are the only ganglia which do not directly arise from 

 ■cells migrating from the overlying ectoderm; on the other hand, 



Pig. 2. — Half -developed embryo of Crepidula adunca, showing the central nervous 

 system and the visceral loop, c, cerebral ganglion and eye; i., intestine; 

 m., mantle cavity; o., osphradial ganglion; p., pleural ganglion; pe., 

 pedal ganglion and otocyst; s., supra-intestinal ganglion; st., sub-intestinal 

 ganglion; v., visceral ganglion. 



they give clear evidence of being products of the cerebral ganglia. 

 At about the same time that the cerebral commissure is forming cells 

 can be seen extending toward the midline from the inner border of 

 each cerebral ganglion. These gradually form an accumulation on 

 •each side of the radular invagination, and in some instances com- 

 pletely invest the radular sac. In the stage represented by fig. 2 

 the ganglia are usually differentiated, though the commissure and 



