36 Journal ok Comparative Neurology. 



ularis. It is possible to suppose that at one time the chief func- 

 tion of the left ganglion was in connection with this parietal 

 eye. The whole apparatus can at present be nothing more than 

 a light-percipient organ. 



b. The thalamus. 



The contribution of neurites from the central grey to the 

 posterior commissure has been mentioned above. The remain- 

 ing cells of the thalamus probably send their neurites caudally 

 without crossing. Both categories of cells serve as part of an 

 end-nucleus for fibers from the striatum. I have not found 

 any special nuclei in the thalamus. 



c. The hypothalamus (Figs. 17, 17 a, 1 8, 19). 



The hypothalamus consists of inferior lobes, corpus mam- 

 millare, and saccus vasculosus. No one of these parts is as 

 large in proportion to the rest of the brain as in Acipenser. 

 The inferior lobes constitute the thick lateral walls, the corpus 

 mammillare forms a thin walled caudal projection, while the 

 saccus forms a thin floor for nearly the whole length of the 

 hypothalamus. The cells of the hypothalamus are not as highly 

 differentiated as in Acipenser, those of the inferior lobes as well 

 as those of the mammillare presenting essentially the same char- 

 acters as do those of the mammillare in Acipenser. 



The inferior lobes have an inner cell zone and an outer 

 fiber zone, which are not so well defined as in Acipenser owing 

 to the peculiar form of many of the cells. The cells are all 

 bipolar with a central process reaching the cavity between the 

 ependyma cells and a peripheral process in the fiber zone. The 

 cells present all gradations between two extreme forms. In 

 one the cell body stands near the cavity and the central process 

 is very short ; in the other the cell body stands far from the 

 cavity and the central process is very long. In the latter case 

 the cell body may lie in the middle or dorsal part of the lobe 

 and the central process reach the cavity at or near the ventral 

 border of the lobe. The process may present all the characters 

 of an ordinary dendrite, and may even be given ofiTas a branch 

 from a large dendrite. The dendrites are not richly branched 

 but consist of a single long axis or of two or more long slender 



