574 LOCV. [Vol. XL 



If these comparisons are well taken, we have in the Petro- 

 myzon, the epiphysis in its highest state of development. Its 

 distal end is enlarged into an eye-like capsule that is more 

 differentiated than their inferior vesicle which represents the 

 pineal eye. This is the reverse of what is true in most 

 Lacertilia, and, on this account the upper capsule of Petro- 

 myzon has usually been compared with the pineal eye of 

 Reptilia and Amphibia. But the fact that the superior capsule 

 in Petromyzon receives its nerve-supply from the posterior 

 commissure, corresponding in this particular with the epiphysis 

 of other animals, has much more weight in determining its 

 homologies than any purely structural resemblances. We are 

 on a better foundation, therefore, in comparing the upper 

 capsule of Cyclostomes to the epiphysis of other animals, than 

 in taking the other position. It may be noted, in passing, that 

 the superior capsule is in the same relative position as the 

 epiphysis, and the inferior one corresponds in position with the 

 pineal eye. 



This view would make the epiphysis eye-like in character, 

 and there are many facts that weigh in that direction. Hill's 

 studies speak strongly for the visual nature of the epiphysis. 

 He shows a high degree of differentiation in the nerve cells to 

 which the nerve from the posterior commissure is distributed. 

 There is also formed in Iguana, Plica, and Phrynosoma an eye- 

 like enlargement at the distal extremity of the epiphysis. 



Between the condition in Petromyzon and that in the 

 Lacertilia many gradations have already been brought to 

 light. 



In Teleosts, the anterior vesicle arises, as in the other 

 groups, but it is transitory, and soon degenerates. It would 

 appear from a recent paper by Ritter, that the anterior vesicle 

 in Phrynosoma is more persistent than in Teleosts, but does 

 not reach the grade of differentiation exhibited in the Lacer- 

 tilia. He gives a figure showing the rare occurrence of the 

 anterior vesicle in an adult Phrynosoma, and although large 

 this capsule is not so highly differentiated as the capsule 

 behind it, but to compensate for this the distal end of the 

 epiphysis is highly developed. In other forms of Lacertilia, 



