author's abstract of this paper issued 

 by the bibliographic service, january 17 



A SKETCH OF THE ORIGIN OF THE CEREBRAL 

 HEMISPHERES 



C. JUDSON HERRICK 



Department of Anatomy, The University of Chicago 



SEVENTEEN FIGURES 



THE PRIMITIVE FOREBRAIN 



From embryological and other evidence it may be assumed 

 that the type ancestral to vertebrates possessed a central ner- 

 vous system in the form of a neural tube with but little com- 

 plication at the rostral end, comparable with that of larval 

 Amphioxus. In response to the peripheral differentiation of 

 eye and nose,, two pairs of lateral evaginations of the walls of the 

 neural tube probably took place early in vertebrate evolution, 

 namely, the optic vesicles and the olfactory bulbs, the first from 

 the betweenbrain (diencephalon) , the second from the endbrain 

 (telencephalon). The embryological evidence suggests that 

 the optic evagination occurred earlier in phylogeny than the 

 olfactory; at any rate, it always precedes in ontogeny. 



In all adult vertebrates the cavities of the original evagina- 

 tions of the optic vesicles have been nearly or quite obliterated, 

 and the retinas and so-called optic nerves very early in vertebrate 

 evolution attained stable and definite form. The cerebral corre- 

 lation center for visual reflexes is primarily in the midbrain, with 

 very small collateral connections (in cyclostomes and Ichthyop- 

 sida) in the thalamus. The progressive enlargement of the 

 thalamic optic centers in Sauropsida and Mammalia is correlated 

 with the elaboration of the cerebral cortex. 



The internal structure of the olfactory bulbs is also in prin- 

 ciple similar throughout the vertebrate series; but the differen- 

 tiation of the secondary olfactory correlation centers at the bases 

 of the olfactory bulbs, which in the lower forms compose prac- 



429 



