58 Journal of Comparative Neurology. 



to the vicinity of the periphery, where the separate i^bres can 

 be traced to processes of the large cells. (Plate IX, Figs. 

 6, 7, and 8.) In like manner the deep cells give off 

 branches to these bundles. It is not easy to be sure 

 whether the second branch of the cell passes directly into 

 an axis cylinder or first forms a reticulum. The later is hypo- 

 thetically more probable, while in some cases the appearances 

 would justify the belief in a direct continuity. The large cells of 

 the medullary portion of the hypoaria seem almost uniformly to 

 give off three main processes and present the appearance of 

 bifurcation or fusion of tracts. They are almost certainly to be 

 understood as the connecting nodes between the dorsal and 

 ventral peduncles. In many cases the actual fusion of two 

 axis cylinders may be observed in one of these cells. In cases 

 where the tracts cross at a right angle the connection is especially 

 evident. Study of this region in turtles shows that the structure 

 is identical although there is no trace of hypoaria in the form of 

 expansions of the base of the brain. The structure of the hypo- 

 aria is so peculiar that it cannot be confused with any other. It 

 amounts therefore to a demonstration of the. truth of our view as 

 to the nature of the hypoaria to discover exactly the same 

 structure in the pes pedunculi region, i. e., the narrow portion 

 of the mesencephalon caudad of the cinereum. Moreover 

 the ventral peduncular fibres can be followed into actual 

 connection with the peculiar bifid cells just mentioned, 

 and the other process passes either to the nidulus rul?er or the 

 large cells connecting with the cerebellum. Still again, we have 

 traced the origin of the hypoaria embryologically as a product of 

 elaborate folding of the brain base consequent on the enormous 

 folding of the roof of the mesencephalon and cerebellum These 

 folds subsequently are nearly obliterated but the hypoaria re- 

 main as evidence of the extrusion of important areas morpho- 

 logically pertaining to the region in front of the ansulate com- 

 missure. The cells arise as products of proliferating epithelium 

 as well as probably migrants from other regions. 



2. 7 he nidiiJus ruber and nidulus niger. In construing the 

 region back of the infundibulum it will be well to keej) in mind 

 what is known and what is suggested with greater or less proba- 

 bility in the case of mammals. We are relieved of one serious 



