I04 UNIVERSITY OF COLORADO STUDIES 



composed of the pia and a single layer of epithelium. The hypophysis 

 is closely applied to this. It is of two parts, a dorsal and a ventral, 

 separated by a constriction (Figs. lo and i6). The anterior part of 

 the dorsal division is of a different nature from the rest of the body. 

 Its exact structure was not clear in my sections. The rest of the organ 

 is of a glandular structure. In the diencephalon, the choroid plexus 

 divides into two main branches; a ventral one extending toward the 

 cavity of the infundibulum, and a dorsal one extending backward into 

 the cavity of the mesencephalon as far as the cerebellum. 



In the roof of the diencephalon, just back of the choroid plexus, are 

 the habenulae, and closely associated with them the supracommissure. 

 Immediately back of the habenulae, the dorsal wall of the brain becomes 

 thinned, till it consists of only a single layer of epithelium. Lying 

 just above this is the pineal body. It is a very much flattened sac of 

 one or two layers of cells. There is a distinct cavity, but it has no 

 connection with the cavity of the brain, though the thinning of the wall 

 of the brain at this place no doubt represents the pineal recess, the 

 primitive lumen of the organ. In the lateral walls of the diencephalon 

 are indefinite gatherings of gray substance, but no distinct optic thalami. 



Mesencephalon. — The anterior boundary of this segment on the 

 dorsal surface is generally considered to be just in front of the posterior 

 commissure, and on the ventral surface, just forward of the origin of 

 the third nerve. As has been found to be the case in other Urodela 

 in distinction to the condition in Anura, there is no dorsal separation 

 of this segment into a pair of gemina. The roof is nearly circular in 

 outline and somewhat flattened. The cavity of the brain in this seg- 

 ment expands laterally to a considerable width, but decreases greatly in 

 height. Backward it narrows again and leaves the segment as a narrow 

 passage. The gray matter of the roof shows separation into two layers, 

 as is common in Urodela. The posterior commissure crosses, deeply 

 intrenched in the gray matter of the roof and sides of this part of the 

 brain. The external layer of white matter of the dorsal and lateral 

 portions of the mesencephalon contains the optic fibers. These course 

 forward and downward over the sides of the diencephalon to form the 

 optic tract (Figs. 11-15). 



