Dendy. — Life-history of the Tuatara. 253 



kindly offered to investigate the detailed development of the 

 brain and the excretory organs, but as the arrangements are 

 not yet complete I do not feel at hberty to publish their 

 names. 



The development of that remarkable organ, the parietal 

 or so-called " pineal " eye, might naturally have been ex- 

 pected to prove of exceptional interest, and these expectations 

 have not been disappointed. This part of the subject I have 

 myself worked out in detail, and my results have been em- 

 bodied in a separate memoir. It will be remembered that in 

 the adult Tuatara the parietal eye, although quite invisible 

 externally, exhibits a higher degree of perfection in structure 

 than in perhaps any other known type. This structure was 

 first described for Sphenodon by Professor Baldwin Spencer. 

 During recent years a large amount of literature has been 

 published by various authors on the structure and develop- 

 ment of the parietal eye in divers types of hzards, and on 

 the corresponding structm-es met with in lower vertebrate 

 types (fishes). As a result of these researches, taken in con- 

 junction with my own observations on the development of the 

 parietal eye in the Tuatara, we may consider ourselves justi- 

 fied in concluding that the ancestors of existing vertebrates 

 possessed, in addition to the ordinary paired eyes, a ixtir of 

 parietal eyes placed side by side on top of the head, and 

 originating as outgrowths of the brain, and perhaps serially 

 homologous with the ordinary paired eyes. 



In existing sharks (Selachians) Locy has shown that the 

 two parietal optic vesicles unite together in the middle line to 

 form the so-called " epiphysis." 



In bony fishes (Teleosts and Amia) Hill has shown that 

 there is also a pair of outgrowths arising in a similar way 

 from the brain, but with more or less displacement. In these 

 fishes, however, the right vesicle alone gives rise to the 

 "epiphysis" of the adult, while the left one separates com- 

 pletely from the brain, and undergoes degeneration. 



In lampreys (Cyclostomes) there is again a similar pair of 

 outgrowths, which suffer displacement in such a manner that 

 the right vesicle comes to overlie the left. Here the right 

 vesicle forms a fairly well organized parietal eye, and the left 

 one the so-called " parapineal organ," and the two together, 

 with the nerve of the parietal eye, form what is usually known 

 as the " epiphysis " in this group. 



At a very early stage in the development of the Tuatara 

 the left parietal eye (optic vesicle) appears as an outgrowth 

 from the fore-brain, slightly to the left of the middle hue. 

 The right parietal optic vesicle appears slightly later, and 

 never attains anything like the same degree of organization as 

 the left one, although exhibiting essentially the same struc- 



