94 Journal of Comparative Neurology. 



consist of pia except in its outer covering. He also describes 

 the same glove-finger process in the sheep and in a cat embryo 

 lOO mm. in length. 



In the embryos of the animals so far mentioned, the cere- 

 bellum has not reached sufficient growth or development to 

 cover in the ventricle and hide it from view. In the dog at 

 birth the vermis only reaches the apex of the ventricle and the 

 evagination of the ventricle is well marked and evident. In the 

 adult dog the vermis reaches over both ventricle and evagina- 

 tion and both are hidden by it. On section, however, the evag- 

 ination can still be made out, but it is flattened so that its roof 

 and floor are in contact in places. The pia covering the ventral 

 surface of the vermis fuses also in places with that covering the 

 oblongata. No openings were found in the evagination. 



The choroid plexus reaches to the end of the evagination 

 but is generally weakly developed, The conditions as found in 

 the dog may be taken as a general type for the majority of the 

 mammalia. 



Other adult types sectioned and studied microscopically 

 are as follows : 



Marsiipialia : Didelphys virginiana. The ventricle has a 

 marked protrusion reaching beyond the vermis, and lying in the 

 subarachnoid space, but entirely closed, Fig. 24. The plexus 

 in the body of the ventricle is weakly developed, while the 

 plexus reaches an enormous size in the lateral recesses, studied 

 by transections and sagittal sections in two animals. 



Rodentia : Lepus cuniculus and Sciurus carolinensis both 

 present the same condition, namely, practically no evagination 

 of the ventricle and a continuous roof. In all, the brains of 

 three rabbits were cut and one squirrel. 



Primates : In all the primates sectioned, with the excep- 

 tion of the Anthropoidae, a peculiar condition was found in re- 

 gard to the caudal extension of the ventricular walls. Appar- 

 ently the pouch becomes compressed between the mesal portion 

 of the vermis and the oblongata and its mesal portion becomes 

 obliterated by the fusion of the roof and floor, while the lateral 

 portions persist along the vallis. Thus the protrusion in their 



