178 Journal of Comparative Neurology. 



In order to bring out the relations of the dorsal commissure 

 system more clearly, a nearly median longitudinal section is 

 given. The plexus was somewhat distorted in the section, but 

 it is apparent that there is a slight depression separating the hip- 

 pocampal from the collosal commissure. 



The callosal fibres are separated from the hippocampal by 

 a pre-plexus which at once reminds us of the relations described 

 in reptiles where coUosum and hippocampal commissure sustain 

 just this relation. It has not been the writer's purpose to enter 

 into details respecting Professor Osborn's theory in subscribing 

 in a general way to the theory that the callosal and hippocampal 

 systems are homodynamous. For example, we do not under- 

 stand the statement that the "Commissura cornu ammonis is not 

 a portion of the fornix system, as has been generally held, but 

 is primitively a portion of the corpus callosum." It is, of course, 

 a mere verbal quibble whether the callosum is part of the hip- 

 pocampal or the hyppocampal commissure part of the callosal 

 system. As a matter of fact the evidence now before us makes 

 it probable that it is the hippocampal commissure which is first 

 developed and in ambhibia this element is greatly in excess 

 over the callossal rudiment. This, too, serves to explain a 

 discrepancy which was noticed by Osborn in the relative date of 

 appearance of the pre-commissure and callosum in mammals and 

 non-mammals. 



The callosiiui and Jiippocampal commissures in Amphibia are 

 very peplexing. It would at first seem that a simple escape is 

 furnished by Osborn's attempt to consider the dorsal commissure, 

 although it lies beneath the ventricle and caudad of the portae 

 as simply an instance of retardation whereby the commissure 

 simply remains in its primative place in the terma, while the 

 brain pushes forward over it. In Menobranchiis, however, he 

 makes the significant discovery that "the upper bundle is com- 

 pletely separated from the lower and crosses the ventricle inde- 

 pendently, so that a fold of the vascular plexus of the pia [sic], 

 which in this genus is greatly developed, passes through the in- 

 terspace." This statement of Osborn we had overlooked, but 

 upon theoretical grounds suspected that the dorsal commissure 



