90 Journal of Comparative Neurology. 



come reduced proportionally to the development of that organ. 

 Moreover in those cases where the callosum lies superposed up- 

 on the fornix there is always a remnant of the plicated roof of 

 the aula between them. 



The amount of gray matter covering the callosal fibres is a 

 matter of no morphological importance. 



In Phrynosovia and other lizards the callosum is in quite a 

 different frontal plane from the precommissure and is quite dis- 

 tinct. Its fibres are derived from the ventricular aspect of the 

 fronto-median cortex. 



In his paper upon the callosum Osborn said, ''the evidence 

 regarding the fornix in Ophidia is not as yet sufficiently clear to 

 be conclusive, but I believe that further evidence will soon be 

 forthcoming to show that there are true fornix tracts in the rep- 

 tilian brain." We think there can no longer be any doubt as to 

 the constancy of the fornix and the hippocampal commissure in 

 all vertebrates above the fishes at least. We subscribe fully to 

 the view that the whole mesal mantle is united with its fellow of 

 the opposite side by a common commissure which is divided in- 

 to a cephalic and a caudal portion, the latter developing earlier 

 because of its relation to the olfactory cortex which is first to 

 become functional. It is not a little strange that Osborn should 

 have failed to find the hippocampal commissure of snakes while 

 recognizing the few and doubtful fibres of the callosum which 

 Meyer thinks simply decussate. 



It is certain that what Osborn called posterior bundle of 

 the callosum is, as he thought probable, " the commissural por- 

 tion of the " Cornu Ammonis." 



In Chelydra we encounter a peculiar development of the 

 commissures which is instructive as to the range of variation. 

 The sections embracing the precommissure show that just dor- 

 sad of it and apparently connecting the two sides of the axial 

 lobe, like the precommissure, is a strong bundle of fibres which 

 turns sharply cephalad and then gradually enters the median 

 cortex. In short, it is the (so-called) callosum, which is dis- 

 placed ventrad and placed in juxtaposition with the precommis- 



