ANATOMY AND DEVELOPMENT. 20J 



account it is called a myelonic eye. In the typical Inverte- 

 brate eye, on the contrary, the retinal cells are differen- 

 tiated from the external ectoderm. 



Comparison of Tunicate Eye zuith the Pineal Eye. 



The Tunicate eye, however, differs essentially from the 

 paired eyes of the craniate Vertebrates in that the lens, as 

 well as the retina, is derived from the wall of the brain. 

 The lens of the lateral eye of the Vertebrates is derived 

 by an invagination of the external ectoderm, which meets 

 and fits in with the retinal cup at the end of the optic 

 vesicle. 



It is, therefore, an extremely interesting fact which was 

 pointed out by Baldwin Spencer, that the Tunicate eye 

 agrees, in respect of the origin of its lens, with the parietal 

 or pineal eye of the Lacertilia, in which the lens is likewise 

 derived from cells which form part of the wall of the 

 cerebral outgrowth which gives rise to the pineal body. 



The pineal body is another of those remarkable rudi- 

 mentary structures whose constant presence in all groups 

 of Vertebrates forms such an eminently characteristic 

 feature of their organisation. It develops as a hollow 

 median outgrowth from the dorsal wall of the brain 

 (thalamencephalon), the distal extremity of which dilates 

 into a vesicle and becomes separated from the proximal 

 portion.* 



For a long time the pineal body was a persistent enigma 



* According to the most recent work on the subject the distal vesicle be- 

 comes entirely constricted off from the primary epiphysial (pineal) outgrowth 

 of the brain, and the parietal nerve does not represent the primitive connex- 

 ion of the pineal eye with the roof of the brain, but it arises quite inde- 

 pendently of the proximal portion of the epiphysis. 



See A. KlinckovvsteOm, Beitr'dge Z7ir Kenntniss des Parietulauges. 

 Zoologische Jahrbiicher (Anat. Abth.), VII. 1893. pp. 249-280. 



