MUSEUM OF COMPARATIVE ZOOLOGY. 197 



stage last described (Figure D., page 190) ; it lies in front and only a 

 little to the right of the abdominal ganglion (Fig. 111). 



The abdominal ganglion (Plate VIII. Figs. 109-111) is less elongated 

 than in the last stage (Fig. 104). It is wedge-shaped, and appears as 

 though crowded in between the two visceral ganglia from behind and 

 above. It is so intimately connected with these ganglia that it almost 

 appears to form a part of .them (Fig. 111). But the presence, between 

 the ganglionic masses, of connective-tissue cells, which reach nearly to 

 the connectives, enables one to make out with some certainty the extent 

 of each of the three ganglia. Since the planes which separate them are 

 oblique to the transverse planes of the body, these boundaries are not 

 always readily seen in cross sections. The right and left visceral gan- 

 glia have no direct commissural nerve fibres uniting them ; they are 

 joined only by such fibres as pass through the abdominal ganglion. 



The buccal ganglia (Plate VIII. Fig. 108, Plate X. Fig. 124) are 

 now entirely separated from the dorsal wall of the radula sac, from 

 which they arose, and are surrounded by a layer of connective-tissue 

 cells. The differentiation of their ganglionic cells is well advanced. 



Summary. 



1. In Limax maximus the whole of the central nervous system arises 

 directly from the ectoderm. 



2. The cerebral ganglia originate in part as a pair of true invagina- 

 tions, one on each side of the body in front of the pleural groove and 

 behind and below the bases of the ocular tentacles. In the course of 

 their development, the neck of each invagination becomes a long, nar- 

 row tube-like structure, which remains open throughout the period of 

 embryonic life. The main part of the cerebral ganglia is formed from 

 cells which are detached at an early period from the deep ends of these 

 cerebral invaginations, or from neighboring ectoderm ; the portions 

 which persist as the walls of the infoldings finally form distinct lateral 

 lobes of the brain. 



3. All the other ganglia originate by cell proliferation from the ecto- 

 derm without invagination. 



4. The ganglia arise separately, and, with the exception of the ab- 

 dominal and mantle ganglia, in pairs, one on each side of the body. 

 Their connection with each other is the result of a secondary process in 

 the development, — the outgrowth of nerve fibres. 



In advanced stages, the central nervous system consists of five pairs 



