6o Journal of Comparative Neurology. 



turn, as we had done the " intra-ventricular lobe," although un- 

 willing to use this term which has a special sense in mammals. 

 Meyer has also noticed the fornix commissure and carefully figur- 

 ed it (Op. cit. , Figs. 33—36) in Iguana. We feel confident that 

 this commissure is, as Riickhard suggested, a true fornix com- 

 missure. In serpents the same fibres lie within that part of the 

 mesal wall which corresponds to the fornix body. The occasion 

 for the separation of these fibers in reptiles is the folding of the 

 brain axis which thrusts the dorsal cortex backward over the 

 diencephalon. 



The statement has been made that the so-called fornix or 

 hippocampal commissure of reptiles cannot be homologous with 

 the organ so named in mammals, because in the former the 

 plexus passes dorsad of it. This is, however, an illusion as the 

 study of these fibers in Ophidia, Lacertilia and Chelonia has 

 fully convinced the writer. In those cases where the plexus lies 

 dorsad of the commissure, it is in the form of a diverticle of the 

 roof cephalad or caudad of the commissure. The fibres them- 

 selves lie in the 7'oof of the aula. 



In amphibia the callosum is very small and, if we correctly 

 identify it, lies for cephalad so that it appears in the same hori- 

 zontal section with the hippocampal commissure. The latter is 

 quite a strong band which after passing along the line of union 

 of the cerebral hemispheres and thalamus turns ventrad and 

 crosses immediately dorsad of the precommissure. We cannot 

 agree with Osborn who identifies this bundle with the collosum. 

 In the first place, its fibres are derived from the caudal part of the 

 cortex and, in the second, they cross caudad of the porta, and, 

 finally, there is a small band of fibres crossing in front of and dor- 

 "^ sad of the porta which better deserves this title. The above re- 

 marks are based particularly upon several good series of Menop- 

 07)ia, McnobrancJiiis, Salavmndra and Rana. The larval sala- 

 mander has the hippocampal tract well-developed while the cal- 

 losum is not apparent at all. 



