No. 3-] THE VERTEBRATE HEAD. 575 



the anterior vesicle reaches a high grade of perfection, while 

 the epiphysis is not well developed. In most cases, however, 

 there is a deposit of pigment in the distal position of the 

 epiphysis. The nerve distribution in that organ is known only 

 in the Teleosts. 



We might carry our comparisons further and inquire 

 whether the structure present in Selachians is to be homol- 

 ogized with the epiphysis or the pineal eye of other forms. The 

 nerve relations are not known in the Selachians and, therefore, 

 we have not, as yet, a satisfactory basis for comparison. In its 

 structure and persistent attachment to the brain roof by a 

 stalk, the outgrowth in Selachians resembles the epiphysis and 

 one would be inclined to say that only the epiphysis is repre- 

 sented in the Elasmobranchs, and that the pineal eye is 

 lacking. Klinckowstrom has offered the ingenious suggestion 

 that in Selachians and birds the second or anterior epiphysis is 

 never fully separated from the posterior, and this is worth 

 thinking about. There is no positive evidence to sup- 

 port it. 



However the above question may be settled, it seems to 

 me that the fundamental features of the morphology of the 

 epiphysis are tolerably clear, and that future work on this 

 subject will be mainly in the direction of amplification and 

 discovery of details. 



The invertebrate homologies of the pineal eye are not 

 known. Spencer supposed it to be homologous with the 

 median eye of Tunicates, but that relation is a strained one. 



Various authors have compared the pineal outgrowths to the 

 ocelli of Arthropods. Leydig compares them to the stemmata 

 of Insects, and shows that there are many resemblances to 

 support the comparison. 



Patten has recently studied the development of the eyes of 

 Limulus, and argues for a homology between the median eyes 

 of these Arthropods and the pineal eye of Vertebrates. He 

 shows the median eye of Limulus to arise by the fusion of 

 paired eye-stalks, giving us a case of undoubted union of origi- 

 nally paired sense-organs. I have claimed a similar occurrence 

 in the epiphysis of Squalus. But it seems to me that the 



