Kingsbury, Brain of Nectmus. 157 



as stated before. The derivation of a few fibers from a nidus 

 of the diencephal was unmistakable (Fig. 29) ; they corre- 

 pond presumably with the fibers from the diencephal described 

 by Bellonci in the Triton. These are in addition to other fibers, 

 some medullated, which pass into the mesal wall from a more 

 dorsal region of the diencephal, — probably the habenas. Fig. 

 29 shows a transection through the cephalic end of the group 

 of cells from which the first named fibers spring. It lies dorso- 

 caudad of the porta. Whether or not these bundles are decus- 

 sational or commissural, or both, could not be determined from 

 the specimens. 



In the larva, as before stated, the dorsal commissure crosses 

 in the floor, separated from the precommissure by several layers 

 of cells. In the just hatched larva the fibers after crossing turn 

 caudad and dorsad in the mesal wall of the hemicerebrum and 

 could be followed to the point where the prosencephal and 

 diencephal join. A derivation at this stage of some of the 

 fibers from the diencephal was strongly suggested but not 

 demonstrated. 



Manifest difficulties attend the homology of this tract with 

 any of the mammalian structures with which it has been com- 

 pared, and the questions of the development, and relation to 

 each other, of the callosum and hippocampal commissure are 

 intimately connected. Herrick ('93,3) has regarded this as a 

 purely hippocampal commissure with possibly callosal ele- 

 ments in it. This homology, however, he apparently made 

 dependent on the recognition of this tract as dorsal. 



It is felt that the concurrent evidence of Diemyctylus, Desmo- 

 gnathus and Nectiirus shows quite conclusively that the fibers of 

 this commissure cross in the terma and do not belong to the 

 roof and simply rest upon the precommissure as thought by 

 Herrick, or cross separately and independently in Nectiirus as 

 stated by Osborn. It is possible that the recognition of this 

 commissure as belonging to the terma might modify Prof. Her- 

 rick's views of the strict homology with the hippocampal 

 commissure. 



The homology that has generally been recognized for this 



