SEPTUM, HIPPOCAMPUS, PALLIAL COMMISSURES 383 



and nucleus lateralis septi (Herrick). I shall hereafter speak of 

 the medial olfactory area as the area parolfactoria and shall 

 use for the two nuclei contained in it the names nucleus parol- 

 factorius medialis and lateralis, respectively. The general fea,- 

 tures of the relations in the medial wall in selachians are shown 

 in figure 10. 



When the medial wall of a reptilian brain is compared with 

 that of the selachian there seems at first sight no great difficulty 

 in identifying the hippocampal formation and the area parol- 

 factoria. The hippocampus has been identified with the medio- 

 dorsal cortex by Meyer ('92), Elliot Smith ('96, '10), Levi ('04) 

 and others. Below the rostral end of the hippocampal forma- 

 tion the area parolfactoria is thickened so as to form a ridge in 

 the ventricle. This is the anterior end of the paraterminal body 

 of Elliot Smith and according to the account given by that 

 author ('03) this body becomes united with its fellow through 

 the lamina terminalis, thus forming the bed of the anterior and 

 hippocampal commissures. The paraterminal body also extends 

 caudad in the medial wall of the hemisphere over the interven- 

 tricular foramen. In certain forms this supraforaminal portion 

 of the paraterminal body continues to the caudal pole where it 

 again unites with its fellow (e.g., in Sphenodon) to form the bed 

 of the commissura aberrans. In mammals it is stated that this 

 paraterminal body surrounds and embeds the entire system of 

 hemispheral commissures (c. anterior, c. hippocampi and corpus 

 callosum). These interpretations are diagrammatically repre- 

 sented in figures 82 and 83. 



The problem before us is to determine what parts of the medial 

 wall of the hemisphere in reptiles and mammals have been de- 

 rived from that definite morphological and physiological unit in 

 selachians which the writer has called the primordium hippo- 

 campi, and what parts have been derived from the area parol- 

 factoria. The examination which follows will show that the 

 paraterminal body of Elliot Smith includes parts of both the 

 area parolfactoria and the primordium hippocampi of selachians 

 and that the bed of the pallial commissures is of different origin 

 and has different morphological relations from the bed of the 

 anterior commissure. 



THE JOURNAL OF COMPARATIVE NEUROLOGY, VOL. 23, NO. 5 



