192 



GASTROPODA. 



present, as we gather from Fol's description, but^the question arises whether 

 they yield only a part or the whole of the cerebral ganglion. From what we 

 as yet know, the latter view is the more probable, and is further rendered 

 possible by the fact that in a Prosobranch (Vermetus) also, the whole of the 

 cerebral ganglion originates from two invaginations (Salensky). These first 

 appear on the velar area as two thickened plates which then sink inwards. 



M 



Xnes 



ect — 



Mies 



FlG. 87. — A and B, transverse sections through two embryos of Helix (Acavus) 

 Waltoni, at different stages (diagrammatic after P. and F. Sarasin). In A, the 

 dorsal, in B, the ventral part of the section Is shown. au, eye ; eg, cerebral 

 ganglia; ct, cerebral tnbes (in B, as the accessory lobes); ect, ectoderm;/, foot; 

 fd, pedal gland ; Ih, body-cavity ; mes, mesoderm-tissue ; oes, oesophagus (in B, with 

 the salivary ducts) ; si, buccal mass (in A , with oesophagus and radular sac) ; t, 

 tentacles. 



The two tubes that arise in this way unite to form the brain and become 

 detached from the superficial ectoderm. The cerebral ganglia were seen to 

 form in exactly the same way in Dentalium (p. 93). It would in any case be 

 interesting to learn in what way this condition may be reconciled with that 

 described for the Pulmonata. The rise of the brain through delamination, 



