154 Journal of Comparative Neurology. 



" At the anterior margin of the left ganglion habenula 

 (the left is much the smaller) a lateral induplication of the 

 membranous roof of the prosencephalon enters the ventricle. 

 This fold is somewhat complicated. The epithelial layer of 

 which it consists forms numerous folds and sacs, into which 

 projections of the membranes and blood-vessels enter. The 

 structure is, in other words, that of a plexus choroideus. 



" Careful study of continuous series of sections indicates 

 that there is nowhere a discontinuity in the epithelial layer. 

 The cavity of the prosencephalon is therefore completely 

 closed. The dorsal lamina of the fold forms the ventral wall 

 of the dorsal sac, and the ventral lamina forms the mem- 

 branous wall of the prosencephalon. It thus appears that 

 the opening of the dorsal sac is asymmetrical. Cephalad 

 from this point the two chambers are separate. The ventral 

 portion of the neural tube at this region consists of thick 

 paired masses, the basal ganglia. The membranous roof 

 bears a system of folds which becomes more complicated. 

 The middle portion projects into the ventricle and forms a 

 sort of falx cerebri. The dorsal wall also circumscribes the 

 dorsal sac laterad and encloses it in a plexiform structure 

 derived from the roof of the prosencephalon. 



" Somewhat cephalad to the base of the olfactory lobes 

 a groove appears upon the dorso-median aspect of the basal 

 ganglia which passes mesad and can be traced directly into 

 the cavity of the olfactory lobe. The median falx of the roof 

 dips deeply into the ventricle of the prosencephalon. The 

 falx extends to the epithelial lamina which connects the two 

 olfactory lobes. Cephalad to this point the membranous roof 

 of the prosencephalon is continued to form paired sacs dorsad 

 to the olfactory lobes. 



" In this structure of the prosencephalon of A. ruthcnus 

 I detect the paired nature of the prosencephalon, which con- 

 sists in the strongly developell falx as well as the mem- 

 branous sacs of its cephalad portion. 



" At the cephalad level of the chiasm two prominences 



