248 FISH. [Vol. X. 



the neurology of the Amphibia, and the exact origin and indi- 

 vidual relations of the fibers is not very definitely known. 

 From an embryological standpoint these morphological differ- 

 ences need not be of much moment. 



Marchand (28) in a series of beautiful figures has shown the 

 development of these commissures in the human brain as seen 

 in mesal section, the first appearance being a fibrous thickening 

 of the terma. This is the proton or anlage of the precommis- 

 sure, and might at the same time be considered to contain the 

 potential rudiments of the fornix and callosum, for as develop- 

 ment goes on, this area as seen from the mesal aspect soon 

 differentiates into a hook-shaped mass by the addition of new 

 fibers due to the increased growth of certain of the cerebral 

 parts. It is quite inconceivable that the fibers forming the 

 callosum should jump across such a gap as the intercerebral 

 fissure ; the presence of the indusium or vestigial cortex found 

 on the callosum in the adult mammals is pretty strong evidence 

 that the new fibers are laid against or insinuate themselves into 

 this original bundle, and in this way the callosum elongates 

 cephalad and becomes thicker proportionately to the develop- 

 ment of the cerebrum. 



In the Desmognathus, where the brain is so elongated and 

 the terma forms so large a proportion of the floor, it is at first 

 sight rather puzzling to determine the homologies of these 

 commissures. The morphological relations are as obscure as 

 they possibly could be ; the precommissure apparently arises 

 from the floor as a short column of transverse fibers, as seen 

 in mesal section, bearing on top of it a smaller bundle, the 

 callosum, or as I interpret it, an undifferentiated forni-callosum, 

 since many of the fibers seem to correspond with those as 

 described by others for the hippocampal commissure passing 

 to the caudal ends of the hemicerebrums, while others, the 

 callosal fibers, are confined more to the mesal walls and help 

 to a slight extent in forming the roof of the paracoeles in 

 this region. The aula lies not only in front (cephalad) of 

 these commissures, but to some extent lies under or ventral 

 to them. If now, instead of stopping at this stage, the 

 growth of the cerebrum should go on in a dorso-caudal direc- 



