480 C. JUDSON HERRICK 



continuation of the corresponding parts of the neural tube in the 

 diencephalon. The ventral part clearly continues the pars ven- 

 tralis thalami, viz., the efferent center lying below the sulcus 

 limitans. The dorsal part is the telencephalic representative of 

 the pars dorsalis thalami, or somatic sensory center above the 

 sulcus limitans. 



(5) The dorso-medial part, or primordium hippocampi. This 

 receives in all vertebrates olfactory fibers of the second or third 

 order at its rostral end and it is connected by association fibers 

 with the ventro-median and dorso-lateral parts. It gives rise 

 to the fibers of the commissura hippocampi, which in all Amphibia 

 is separated into a commissura pallii anterior and a commissura 

 pallii posterior. The columna fornicis passes from it to the 

 hypothalamus by the usual coarse, from the rostral end of the 

 primordium dorsal and rostral to the interventricular foramen 

 and anterior commissure, thence through the lamina terminalis. 



In reptiles (and possibly to a very slight extent in Anura) cor- 

 tex hippocampi is differentiated within this part. In mammals 

 it becomes the hippocampus. In cyclostomes, and perhaps in 

 some other fishes, the dorso-median part of the hemisphere is 

 connected by a massive bridge of grey matter with tissue in the 

 epithalamus closely associated with the habenula. In Amphibia 

 and Amniota, this connection is lost. Nevertheless, it seems prob- 

 able that the primordium hippocampi is topographically the 

 telencephalic extension of the epithalamus. The establishment 

 of different functional connections of these structures has led 

 to a wide divergence of form and significance. 



The course of the commissural fibers of the primordium hippo- 

 campi shows remarkable variations. In mammals these fibers 

 pass forward above the interventricular foramina to cross in or 

 above the lamina terminalis. In ganoids and teleosts, they go 

 below and behind the foramina. In Selachians, Amphibia and 

 some reptiles they separate into the commissura pallii anterior 

 and posterior. In the amphibians, the anterior part passes for- 

 ward behind and below the foramina to cross in the lamina term- 

 inalis and the posterior part backward by way of the stria medul- 

 laris to cross in the superior commissure. In reptiles the anterior 



