264 PROCEEDINGS OF THE ACADEMY OP [1884. 



the median line, but here they are separate, as vaay be seen 

 by a close external examination, and verified by transverse 

 sections. This homology is confirmed by the stud}- oi the fore- 

 most of the transverse commissures. In front of this space 

 rises the reddish body, which has been generally mistaken for 

 the epipliysis. In the natural state this body is not very promi- 

 nent, but as soon as the ventricles collapse, it is thrust conspicu- 

 ously upwards. The ventricular collapse is also the occasion of 

 an artificial dorsal furrow in the optic lobe, which is here abso- 

 lutely unpaired. Upon the ventral aspect of the brain we again 

 observe two transparent areas. One resembles a long slit in 

 front of the optic chiasma, and is found to be a portion of the 

 lamina terminalis. The other is due to a thinning of the floor of 

 the infundibulum, and is seen immediately in front of the 

 hypoph3'sis. The hemispheres are closely applied to each other, 

 but have no structural union. The cerebellum is slightly over- 

 hung by the optic lobe. 



The proportions of the various segmental parts are very 

 similar to those of Menobranchus, and this seems to accord with 

 the similarity'' of the proportions in the head, body and limbs of 

 these animals. 



Internal Structure. 



A natural introduction to the internal structure would be 

 a description of the walls and cavities of the various segments, 

 but it happens that the boundaries of these segments can only 

 be determined after we settle upon the relations of the parts 

 which compose them, so, until some of the details have been 

 investigated, this description must be postponed. In general, 

 the brain is a tube forking in front into the paired lobes and 

 cavities of the hemispheres. 



The Ependyma and Pia Mater. — The pia mater closely invests 

 all the brain surfaces and sends numerous nutrient vessels into 

 its walls. It envelops all parts of the brain, with the exception 

 of the hypophysis, which lies external to it (fig. 4), so that the 

 pia actually separates the h3rpoph3'sis from the floor of the 

 infundibulum and sends in numerous smaller vessels between 

 the epithelial tubes which constitute this body. This relation is 

 not true of the posterior lobe of the hypophysis which is a 

 development of the brain-wall and is surrounded by the pia, the 

 anterior lobe as is well known, arising from the oral epithelium. 



