250 FISH. [Vol. X. 



Proteus. Herrick has confirmed this in the Necturtis and finds 

 it a matter of congratulation, because the plexus being a 

 diverticle of the roof and separating the commissures in this 

 way, he recognizes " of necessity that the commissure is mor- 

 phologically dorsal " and belongs to the roof and not to the 

 floor of the ventricle. I cannot but feel that this arrangement 

 in Nectitnis is of but little or no morphological importance, and 

 is only an exaggerated variation of the conditions found in 

 allied forms. We find that in some AmpJiibia these commis- 

 sures are practically one, while in others they may have grown 

 more or less apart, but in no case that I know of to such an 

 extent as is described for the adult Nectiirus. In all these 

 other forms then, the callosum arises at the same time or as a 

 part of the precommissure from the floor, or more properly the 

 terma, of the ventricle. A study of a series of transections 

 through the head of a young Necturus 38 millimeters in length, 

 shows that there is no such separation of the callosum as is 

 described for the adult ; there are some cells interpolated 

 between it and the precommissure as in the DesmognathuSy but 

 not the least indication of a separation by means of a cavity. 

 Observations upon the brain of an adult Necturus cut into 

 sagittal sections likewise showed a non-separation of these 

 commissures except by a simple cellular layer. Although the 

 plexus may have arisen as a diverticle of the roof and later 

 interpolated one of its branches between these commissures, 

 this fact need not necessarily make the commissure a part of 

 the roof, for we might on similar grounds call the dorsal or 

 caudal wall of the infundibulum a part of the roof, because in 

 some instances a branch of the plexus extends into its cavity. 



Herrick (17), PI. XV, Fig. 5, and PI. XVIII, Fig. 5, shows 

 in the Necturus, as the callosum, a slight projection caudad of 

 the mesal walls of the cerebrum and represents fibers appearing 

 from it in these same mesal walls or intra-ventricular lobes. 

 In a later paper (The Callosum and Hippocampus in Marsupials 

 and Lower Brains, Jour. Comp. Neurology, III, pp. 176-182. 

 2 plates, 1893), he reiterates this statement, but also adds that 

 " it might still be considered possible that callosal elements 

 were bound up in the larger hippocampal commissure." Mrs. 



