SEPTUM, HIPPOCAMPUS, PALLIAL COMMISSURES 373 



The work of the writer has shown that the telencephalon of 

 cyelostomes is truly primitive as compared with that of other 

 fishes. It is believed that the more primitive selachians are the 

 nearest existing allies to the common ancestors of fishes, amphib- 

 ians and reptiles. Among the reptiles those which are regarded 

 by paleontologists as nearest the line of mammalian descent are 

 the chelonia. 



The writer's reasoning has been that an understanding of the 

 morphology of the mammalian and human brain must rest on 

 a clear knowledge of the steps in its evolution and of the forces 

 (habits, mechanical factors, etc.) which have been concerned in 

 that evolution. For this purpose we must be able to give a 

 consistent account of the evolution of the brain through the 

 chief groups of vertebrates which stand nearest the line of de- 

 scent of higher mammals: the cyelostomes, selachians, dipnoi, 

 chelonia, monotremes, marsupials, and certain mammals. With 

 this in view the writer, having studied the telencephalon in 

 cyelostomes and selachians, wishes to present here some con- 

 tributions upon reptiles and mammals. Further studies of dip- 

 noi, as alTording a bridge between selachians and reptiles, are 

 greatly to be desired. 



The reason for the above statement of the writer's method 

 of approach to this problem is that the facts thus far observed 

 in the telencephalon of primitive vertebrates do not agree with 

 the accepted views regarding the mammalian brain. The diffi- 

 culties have to do chiefly with the relations of the paraterminal 

 body, lamina terminalis, commissures and hippocampus. In 

 selachians the pallial commissures are imbedded in a massive 

 roof (called by the writer the primordium hippocampi) and cross 

 through the lamina supraneuroporica, wholly independent of the 

 lamina terminalis and the paraterminal body. The fiber con- 

 nections of the body called primordium hippocampi are so char- 

 acteristic and the line of demarcation between it and the para- 

 terminal body is so clear, that there can be no question of the 

 individuality of the primordium hippocampi in selachians and 

 cyelostomes. The only doubt which has arisen in the use of 

 this term has been whether the body in question may not give 



