482 C. JUDSON HERRICK 



primordium hippocampi lies chiefly in the posterior pole and the 

 long septum ependymale cuts off the dorsal path to the lamina 

 terminalis ; accordingly the commissural fibers take the same path 

 as in teleosts. But this by no means implies a genetic relation- 

 ship between teleosts and Amphibia or that the Amphibia are 

 more closely related to the teleosts than they are to the reptiles. 

 From the standpoint of evolution our interest centers not in the 

 variations of these commissural fibers, which arise late in the on- 

 togeny and have small phylogenetic significance, but rather in 

 the changes in position of the grey masses from which they spring, 

 which in this case concern merely their relative cephalo-caudal 

 position — a factor easily varied in the course of phylogeny by 

 changes in the composition of the sensori-motor reflex pattern or 

 action system of the species. 



In reptiles and mammals the five parts of the cerebral hemisphere 

 as above summarized have very different histories. The olfac- 

 tory bulbs suffer no fundamental change, nor does the pars ventro- 

 medialis save for the differentiation within it of special nuclei, 

 chiefly under the influence of the descending columns of the fornix 

 and of the corpus striatum. Thev entro-lateral and dorso-lateral 

 parts give rise to the corpus striatum and pyriform lobe, which 

 remain in very intimate relation from the beginning to the end of 

 the phylogenetic history. 



The nervus terminalis has not been considered in this paper 

 for the reason that we still lack sufficiently precise data to effect 

 its morphological interpretation. Previous descriptions indicate 

 that it is related probably with the precommissural body. Mr. 

 P. S. McKibben in this laboratory has in preparation a report on 

 the central relations of this nerve in urodeles, in which he finds it 

 related with the whole of the pars ventro-medialis hemisphaerii, 

 still more extensively with the nucleus preopticus and also with 

 the hypothalamus. But we have as yet no sucffient data for 

 a determination of its functional connections and therefore its 

 morphology remains obscure. 



The relations of the amphibian dorso-medial and dorso-lateral 

 parts to the cerebral cortex will be considered on a later page; but 

 first we must recur to the relations of the precommissural body. 



