152 Journal of Comparative Neurology. 



in which the nerve cells extend to the exterior, generally asso- 

 ciated together in small clusters. It is believed that the rela- 

 tions in this region cephalad of the terma are important in view 

 of the relations in the Reptilia and the possible homology of 

 the hemiseptum (septum lucidum). 



ParapJiysis. This is quite large in Nechwiis, and enveloped 

 in and distorted by the supraplexus ; it extends cephalad closely 

 applied to the roof of the skull. The communication with the 

 cavities of the brain in the adult was constant, ageeing in this 

 respect with Dianyciyhis. In Aviblystovia ('92) the communica- 

 tion with the cavities of the brain was found to become inter- 

 rupted in larvae twelve millimeters in length. ^ In the just 

 hatched larva {Ncchinis) the weak development of the supra- 

 plexus leaves the parts in a simpler condition, and the paraphy- 

 sis appears as an elongated sack lined by a single layer of cells, 

 (Fig. 8) and communicating with the encephalic cavities by a 

 narrow neck. 



In the greatly elongated terma, which in NecUiriis forms in 

 part the floor of the brain, immediately caudad of the portas, 

 is a marked shelf-like elevation in which cross from side to side 

 two bundles of fibers, the more ventral, the precommissure, and 

 the dorsal, the callosum. generally recognized as such hitherto. 

 The correctness of this homology seems doubtful, and in order 

 not to prejudice the question in the outset, it will be here 

 spoken of as the dorsal bundle or commissure. There has been 

 great difference in the statements made concerning the relations 

 of this bundle to the precommissure in Nechiriis. Osborn i^'^Q) 

 in his memorable paper on the "Origin of the Corpus Callo- 

 sum ',' mentions and figures the dorsal bundle, — his callosum — 

 as completely separated from the ventral by a part of the cavity 

 and a fold of the plexus, and he further found the same condi- 

 tion in Proteus. Herrick ('93, 3) speaks as follows of the rela- 

 tions of these commissures in Necturus : 



1 In his paper on the development of Amblystoma, which has just appeared 

 ('95) Eyciesheimer reiterates the statement of the obliteration of the proximal 

 portion of the cavity of the paraphysis, and a discussion of the significance of 

 the organ is given. 



