230 W. BALDWIN SPENCER. 



tion of a creature whose skin has become opaque, and to whom 

 an eye within the brain has become useless, it would be ex- 

 tremely difficult to say with certainty;^ it is, however, worth 

 while calling attention to the fact that the epiphysis in very 

 early stages in its development in Bufo cinerea resembles 

 the Tunicate eye before the appearance of refractive elements, 

 whilst subsequent loss of pigment and evagination transforms 

 it into the epiphysis of the adult. 



If there be any truth in the above hypothesis it follows that 

 we must start with a form which may be regarded as the 

 common ancestor of present Tunicata and the higher Chor- 

 data ; in this, which closely resembles an embryonic Tunicate, 

 certain cells of the dorsal wall of the neural cavity are 

 specially elongated and bear pigment at their internal ends, 

 just as in the embryo Tunicate eye and Anuran epiphysis. 

 From this point development leads in two directions — (1) to 

 the highly developed internal eye of present Tunicata with 

 its secondarily developed refractive structures, and (2) by 

 evagination and loss of pigment to the epiphysis of higher 

 Chordata. Subsequent diflFerentiation in the latter results 

 in the formation of a distal vesicle united to the brain roof by 

 a stalk, at first hollow and afterwards solid, whilst finally the 

 distal vesicle becomes modified into the pineal eye. 



The evolution of the epiphysis is represented diagrammatically 



' It will be seen tbat this differs from the suggestion of Professor Lankester 

 that the internal eye of Tunicates by evagination forms the Vertebrate eye. 

 In the first place 1 suppose the evagination to give rise to the epiphysis, 

 subsequent differentiation of the distal vesicle of which gives origin to the 

 pineal eye. Secondly, I assume the development of the Tunicate eye and the 

 epiphysis out of an ancestral form common to Tunicata and the higher 

 Chordata, development taking place along two different lines and being possibly 

 connected with the transparency of the one and the opacity of the other form. 

 At the same time it may be pointed out that it is possible that the paired eyes 

 may be formed by evagination of paired internal eyes similar to the one which 

 becomes transformed into the epiphysis. The vesicles giving rise to the 

 paired and pineal eyes are precisely similar to each other, and may have 

 originated in the same way, the two types of eyes being entirely the result of 

 the development of secondary structures. 



