MORPHOLOGY OF THE FOREBRAIN 481 



part passes forward above the foramina to cross in the lamina 

 terminalis and the posterior part crosses directly through the 

 velum transversum. In selachians the anterior part passes for- 

 ward above the foramina to cross above the lamina terminalis 

 and the posterior part passes backward to cross in the superior 

 commissure, as in Amphibia (Johnston, '10). 



The peculiar aberrant courses shown by a comparative study 

 of the commissura hippocampi (and many similar illustrations 

 might be given) suggest that these fiber systems are in the phylog- 

 eny functionally determined and that the particular pathways 

 selected by the fibers in passing from origin to termination are 

 variable depending in part on the mechanics of ontogenetic and 

 phylogenetic development. 



In this connection it should be borne in mind that the commis- 

 sural fibers of the primordium hippocampi develop late in the 

 ontogeny after the chief structural differentiation of the brain 

 wall has far advanced. In macrosmatic species, like the sela- 

 chians, where the secondary olfactory nuclei are greatly enlarged 

 at the rostral end of the hemisphere (particular^ the ventro-me- 

 dian nucleus) the primordium hippocampi attains its greatest 

 development at its rostral end and some commissural fibers take 

 the shortest path across the area of fusion of the primordia in the 

 lamina supraneuroporica. In reptiles the same factors operate 

 to some extent and in addition the rapid backward growth of the 

 posterior poles of the hemispheres and the consequent early de- 

 velopment of the posterior chorioidal fold tend to impede the 

 subsequent growth of fibers from the hippocampus downward 

 into the lamina terminalis behind the foramina. Accordingly they 

 grow forward above the foramina into the lamina terminalis. 

 But in some cases (lizards) where the anatomical configuration is 

 favorable some of these fibers from the posterior pole find a shorter 

 path and cross in the velum transversum to form the commissura 

 pallii posterior. In ganoids and teleosts, on the other hand, the 

 centers containing the primordium hippocampi lie far back in 

 the telencephalon medium and their commissural fibers grow for- 

 ward below the ventricle into the lamina terminalis by the shortest 

 path. Similarly in the urodeles, especially the early larvae, the 



