IV 



Journal of Comparative Neurology, 



For the human brain the following averages are given 

 Syi to 6;^ months — O. idex, 18.8; P. index, 28.5. 

 6^ to 7)^ months — " 



7j!^ to 8j^ months — 

 Full time foetuses — 

 First .12 months — 

 4 to 5 years — 



II to 15 years — 



Adults — 



18.6 

 20.7 

 20.8 

 22.3 

 23.2 

 20.8 

 21.2 



24.7. 

 24.1. 



25.7. 

 25.6. 

 24.2. 

 27.4. 

 25-5- 



For apes the following table is given : 



Orang — O. idex, 23.2; P. index, 21.3. 



Chimpanzee — 

 Homadryas — 

 Cynocephalus — 

 Mangaby — 



Macaque — 



Cercopithecus — 

 Cebus — 



24.2 ; 



29-5; 

 29.7; 



30.5; 

 310; 

 32.9; 

 > ^- 1 \ 



19.9. 

 20.5. 



22.6. 

 24.1. 

 19.0. 

 19.0. 

 20.6. 



The low parietal index and high occipital index are instructive 

 features. In low apes there is an enormous increase in the occipital 

 portion of the border; whereas, in the high apes, the amount of in- 

 crease is smaller. The relative shortness of the parietal border in 

 high apes is due to the relative increase of the occipital and frontal 

 borders. In low apes the reduction of the parietal portion is entirely 

 due to the great size of the occipital lobe. 



The following summary is reproduced verbatim : 



1. At an early period in the development of the cerebral hem- 

 isphere a series of deep infoldings of its thin walls make their appear- 

 ance On the exterior of each hemisphere these show in the form of 

 sharply-cut linear fissures. 



2. Certain of these fissures are permanent; the great majority 

 are transitory. 



4. The transitory fissures, with two exceptions, have disap- 

 peared by the time the corpus callosum is fully formed. 



4. A deficiency of the corpus callosum is associated with a per- 

 sistence of the temporary fissures. 



5. The temporary fissures indicate an important stage in the 

 growth of the crebrum, and are apparently associated with the map- 

 pmg-out of the occipital lobe. 



6. A quadrupedal pause in the growth of the cerebrum brings 

 the skull capsule into antagonism with the growth of the hemispheres, 

 and in consequence, the wall of the cerebrum is thrown into folds. 

 These folds disappear as the occipital lobe assumes shape, owing to 



