Blake, Roof of the Fourth Vejiiricle. 95 



forms obtains the shape of a Y whose arms project caudally in 

 the valHs, lying between vermis, hemisphere of cerebellum and 

 the oblongata. 



Hapalid(2 : Hapale. Two brains sectioned. In both the 

 caudal pouch was closed and had lateral extensions, in one ex- 

 tending caudad as far as the caudal extremity of vermis, in the 

 other not so far. 



Cercopithccid(2 : Macacus cynomulgus. Pouch was pres- 

 ent with lateral extensions, but closed. In the remaining old- 

 world monkeys sectioned, namely, Macacus rhesus (two speci- 

 mens), Macacus memestrinus and Cynocephalus anubis, the 

 conditions were the same with the exception that the extremities 

 of the lateral extensions were open. Thus there was a double 

 metapore formation. These openings, however, were very small. 

 Through them the choroid plexus reached into the subarach- 

 noid cavity. 



No other adult forms were sectioned microscopically. Sev- 

 eral adult brains, however, were examined by gross methods. 



As far as could be determined by this method the conditions 

 found in the sheep, ox and cat, resembled those in the dog. 



Two brains of the chimpanzee (T. niger) were examined. 

 In one there was a metapore formation exactly similar to that 

 found in man, with a caudal extension of the choroid plexus on 

 the vermis. The other exhibited a closed pouch of only mod- 

 erate extent the choroid plexus being very short, resembling 

 the condition exceptionally found in man. 



These observations corroborate those of Wilder who found 

 a metapore in the chimpanzee, gorilla, orang and several old- 

 world monkeys. 



In the Anthropoidae the vermis is relatively small as in 

 man and it does not occlude as a rule the caudal extremity of 

 the ventricle. 



In the Cercopithecidae, as before stated, the vermis does 

 project caudad to the extremity of the ventricle and its pia fuses 

 more or less with the pia of the oblongata. The caudal protru- 

 sion thus becomes compressed, but the tendency to metapore 

 formation is present in the lateral portions of the sac in the ma- 



