Johnston, Morphology of the Head. 243 



pair disappears while the other moves nearer the middle, or 

 whether one or the other process may occur in different forms. 

 More important for the question of segmentation is the fact 

 stated above that one epiphysis is situated behind the other. 

 Since the anterior one has its stalk and center in the diencephalon, 

 it may be assigned without hesitation to the diencephalic (iii 

 neuromere and head segment 3. If the posterior one is one 

 segment farther back it must fall into the anterior of the two 

 mesencephalic neuromeres (iv) and head segment 4. This is 

 consistent with the fact that its fibers pass (apparently) into the 

 optic lobes. It must be noticed, however, that since the stalk 

 of this second epiphysis stands between the superior and pos- 

 terior commissures its assignment to neuromere iv implies that 

 the anterior limit of the mesencephalon is not the posterior com- 

 missure, which has been taken as the most conv^enient mark. 

 The limit must lie between the two epiphyses, some distance 

 in front of the posterior commissure. 



On the supposition that the epiphyses are homologous with 

 the lateral eyes, and the evidence is all in favor of this, the 

 writer would consider them to be metamorphosed cutaneous 

 ganglia belonging to the third and fourth neuromeres and head 

 segments. In the case of the epiphyses, their wholly dorsal 

 position, the direct entrance of the fibers into dorsal centers 

 without a chiasma, and the presence of the cutaneous center 

 (tectum) in the segment of the second epiphysis all indicate that 

 the cutaneous centers have remained in the brain. It seems 

 probable that the centers for the first epiphysis have degener- 

 ated with the reduction of the organ. This view is suggested 

 by the fact that in Petromyzon, where the organ is functional, 

 the left ganglion habenulae divides into an anterior part which 

 comes into connection with the epiphysial vesicle, and a pos- 

 terior part which receives the tractus olfacto-habenularis through 

 the superior commissure. The finding of these facts in 

 Petromyzon changed the view which I had formed from the 

 study of Acipenser, namely that the ganglion habenulae had 

 changed its function after the reduction of the epiphysis. The 

 structure and relations of the ganglia habenulae can be better 



