26 Professor W. Baldwin S})encer [Jan. 28, 



former, that is, there must have been a stage passed through in which 

 the internal eye of the Tunicate-like ancestor was undergoing 

 evagination, and during which period it could not function as an 

 organ of vision, but it is possible that the internal eye of the small, 

 transparent form became changed into the externally-placed epiphysis 

 of the larger and more oj)acLue animal which was gradually evolved 

 out of the Tunicate-like ancestor. By fui-ther and secondary 

 modification the epij)hysis gave rise to a distal vesicle and a proximal 

 pineal stalk, whilst still later again the former became modified into 

 the pineal eye. 



Henri de Graaf in his memoir on the development of the epiphysis, 

 mentions as a curious fact in its formation in Bufo cinerea, that it 

 originates as a thickening of the brain roof and that there is a small 

 mass of pigment present for some little time at the inner ends of the 

 cells forming the thickening ; now this is exactly the development, 

 according to Kowalevsky, which is passed through in early stages 

 during the formation of the eye of a larval Tunicate ; in the latter the 

 eye comes subsequently to lie within the brain, still more pigment 

 being developed and a lens formed, whilst in Bufo the pigment dis- 

 appears, the thickening forms into an evagination, and the e2)iphysis, 

 as present in all higher Vcrtebrata, is gradually formed. It is quite 

 possible, however, that this may present us with some of the steps 

 passed through in the gradual development of the epiphysis out of 

 the internally placed eye of the Tunicate-like ancestors of living 

 Vertebrata. We may perhaps go even a step further and find in these 

 low vertebrate forms the structures which by a similar evagination 

 have given rise to the optic vesicles out of which by a later modifica- 

 tion were developed the paired eyes. In one form of Tunicata — the 

 galps — are found in the brain (that is the persistent anterior ex- 

 tremity of the larval nerve-cord), not one, but three eyes, of which 

 one is median and two are lateral ; we may perhaps be here presented 

 with the organs which have gradually given rise, during the change 

 from a transparent to an opaque animal, to the three vesicles which 

 in all Vertebrata, save the Tuiiicata and Amphioxus, now form the 

 single dorsal epiphysis and the paired, lateral, optic vesicles. 



At the present time the median pineal eye is found only in lizards, 

 and even here it is but in a rudimentary condition ; it is always 

 intimately associated with the presence of the parietal foramen ; if the 

 foramen be present the eye is found in a more or less highly developed 

 state, if the foramen be wanting the eye is absent. It is curious to 

 note that the foramen is a marked characteristic of extinct forms of 

 vertebrate life — of the Labyrinthodonts of Palaeozoic, and the 

 Saurians of Mesozoic times ; so far as can be told it is simply formed 

 to allow the distal extremity of the ei^iphysis to pass through the 

 skull roof, and thus to enable the pineal eye to be placed upon the 

 surface of the head. We are probably right in assuming that the eye 

 was most largely developed in these extinct forms, and thus it is 



