No. I.] DESMOGNATHUS FUSCA. 249 



tion analogous to that of higher forms, then would the terma 

 and its commissures pass or bend in a cephalo-dorsal direction 

 through an arc of ninety degrees and the morphological relations 

 would approximate to those of higher vertebrates. 



The difficulty of homologizing these parts has been largely 

 due to the inadequate or inaccurate limitations put upon the 

 boundaries of the terma. To homologize with higher forms 

 and with a due regard for the morphological front or cephalic 

 end of the brain, it seems to me its most ventral or caudal 

 boundary should be at the chiasma and the dorsal or cephalic 

 boundary where it meets the supraplexus. Wilder (58), with 

 the mammalian brain as a basis, has proposed the terms proso- 

 terma for that portion dorsal to the precommissure, and dia- 

 terma for the remaining portion which he also regards as 

 properly belonging to the floor. In the absence of any special 

 line of demarcation in the terma itself or of the aula, such a 

 division seems unnecessary, for there is no apparent change in 

 the morphology except that the area allotted to the aula is 

 somewhat increased. These commissures are all developed in 

 the terma and assume certain relations to the other parts 

 according to the development of the cerebrum. When the 

 cerebrum has developed to such an extent as to overlap 

 the diencephal and mesencephal, it must obviously cause the 

 terma to bend in a dorso-caudal direction and markedly change 

 the relations of the parts in this region; so that in the 

 Amphibia, where a considerable portion of the terma is ventral, 

 the commissures seem to spring from the floor ; but if the 

 terma be bent upward and backward (dorso-caudad), allowing 

 for a corresponding and perhaps different rate of change 

 for the cavities, the homology would be much more evident. 

 Figs. 20-24 represent the position and relations of these parts 

 in the Desmognathus, CryptobrancJms, frog, turtle, and bird. 

 These forms make a good series phylogenetically for the illus- 

 tration of this point and for the exemplification of what has 

 just been said. 



Osborn has discovered that the callosum in Necturus is 

 entirely separated from the precommissure by a fold of the 

 nlexus and he states that a similar condition exists in the 



