Herrick, Callosinn and Hippocampus i)i Lotvcr Brains. 179 



would prove to belong to the roof and not to the floor of the 

 ventricle. We, therefore, had introduced the accompanying 

 drawing (Plate IX, Fig. 5.) before noticing that Osborn's Fig. 

 14 is substantially identical. We have never found the dorsal 

 commissure so completely separated as represented by the latter, 

 but always in juxtaposition with the floor ; it was, therefore, a 

 matter of congratulation to find a clearly defined continuous 

 film of epithium separating from the subjacent pre-commissure. 

 The series being continuous and faultless and doubly stained 

 with haematoxylin and fuchsin to differentiate epithelial from 

 nervous elements the evidence is conclusive. Knowing, as we 

 now do, that the plexus is but a diverticle of the roof we recog- 

 nize of necessity that the commissure is morphologically dorsal. 

 The ventricle, which would otherwise be occluded, is in indirect 

 communication by means of the paraphyseal vesicles. The pre- 

 paraphysis lies in front of the commissure, the post-paraphysis 

 behind it — in other words the relations are as in serpents in that 

 respect. The relation of the fibres from this bundle is • well 

 shown in Fig. 5., Plate XVIII, of the present volume. The 

 great mass of the fibres pass to those caudal and mesal walls of 

 the hemisphere which may be confidently homologized with the 

 hippocampus. Some of the fibres which lie in the intraventri- 

 cular region certainly continue forward into the olfactory bulb, 

 as shown by Bellonci, and hence should be homologized with 

 the strice Lancisii. On the basis of these facts it would seem 

 most natural to conclude that the same relations once existed in 

 Anura. If this could be admitted the caudal position of the 

 commissure would be explained, as also in the similar Chelonia, 

 If the callosum exists in Amphibia it would be expected to lie in 

 the terma cephalad of thepre-paraphysis where a few fibres have 

 been noticed in Menobranchus (see plate XVIII, Fig. 5.) 

 though it might still be considered possible that callosal ele- 

 ments were bound up in the larger hippocampal commissure. 



Reptilia. It is remarkable that the hippocampal commis- 

 sure is well developed in terrestrial reptilia-Ophidia, and Lacer- 

 tilia, in which the olfactory apparatus is largely developed, but 

 absent or small in aquatic forms, Crocodilia and Chelonia. In a 



