lOO UNIVERSITY OF COLORADO STUDIES 



tricles extend well forward into the olfactory lobes, and their forward 

 extremities curve inward so as nearly to reach the mesial surface. 



In the hemispheres, the lateral ventricles extend the length of the 

 segment. They communicate with the anterior part of the third ven- 

 tricle by the slit-like interventricular foramen (foramen of Monro) 

 (Figs. 11-16). The lateral ventricles are surrounded entirely by gray 

 matter, and a conspicuous spur, known as Ammon's horn, extends 

 forward from the posterior end of each hemisphere. The pallium is 

 well developed, comprising quite a thickness of gray matter. The 

 striated bodies appear as thickenings on the ventro-median walls of 

 the hemispheres. On the dorsal median line, in the angle between the 

 caudal ends of the hemispheres, appears the choroid plexus of the third 

 ventricle, often called the supraplexus. It drops down into the brain 

 and forms the anterior wall of the third ventricle above and forward 

 of the lamina terminalis. It sends branches forward and backward. 

 The forward extension reaches far into the lateral ventricles. 



The lamina terminalis in Amphibia is generally considered as extend- 

 ing from the optic chiasma forward and upward to the choroid plexus. 

 Immediately back of the interventricular foramen, there arises from 

 the lamina terminalis an elevation which comprises the fibers of two 

 commissural bundles (Figs. 10 and 16). The ventral bundle is the 

 anterior commissure, the dorsal one the so-called corpus callosum. In 

 Plethedon these bundles are separated at the median line by a single 

 layer of cellfe, apparently continuous with the gray matter of either side. 

 Almost immediately on either side, the dorsal bundle bends abruptly 

 upward and the ventral one more slowly downward, so that they soon 

 become separated by a considerable amount of gray matter (Fig. 5). 

 The fibers of the upper tract take a course upward and forward to form 

 the posterior and dorsal walls of the interventricular foramen and almost 

 immediately spread out on the median and dorsal walls of the lateral 

 ventricles (Fig. 16). 



The question of the true homology of the dorsal bundle is a much 

 debated one. It occurs in other Urodela in much the same relations as 

 in Plethedon, and in Anura the conditions are essentially the same. It 

 has quite generally been considered to be the homologue of the callosum. 



