PAEIETAL EYE AND ADJACENT OEGANS IN SPHENODON. 113 



Sphenodon is generally recognised as being the most 

 ancient type of reptile still existing, and it might there- 

 fore well have been expected that the parietal eye in this 

 animal would be better preserved than in any other type. 

 Thanks to the classical works of Baldwin Spencer (33) and 

 de Graaf (15), this has long been known to be the case in 

 the adult, and I hope in the present memoir to be able to show 

 that its development has also undergone less modification than 

 in other reptiles, a fact which enables us to make a remark- 

 ably close comparison with the development observed by Hill 

 in fishes, and presents us with an important clue to the phylo- 

 genetic history of this remarkable organ. 



As on so many previous occasions, I have to tender my 

 most sincere thanks to Professor G. B. Howes, LL.D., F.R.S., 

 for his kindness in revising the proof sheets of these pages in 

 my absence from England, as well as for much valuable advice 

 and assistance in the matter of literature. I am also greatly 

 indebted to Dr. Gaskell, to Professor J. E. Reighard, and to 

 Professor W. B. Benham for sending me works which I was 

 unable to obtain in Christchurch. 



The letters employed to designate the different stages are 

 the same as those of which I have already made use in my 

 general account of the development. 



2. Systematic Account of Observations on Sphenodon 



punctatus, 



Stage K. 



The first indication of the parietal eye appears at the stage 

 in the development which I have called K, comparable with a 

 chick of about two days. The fore-brain is already bent 

 downwards and backwards, so that the mid-brain lies at the 

 anterior extremity of the body. The optic vesicles^ of the 



^ lu the summary of my results recently seut to England I have, by an 

 unfortunate slip, stated (10) that the parietal eye first appears shortly after 

 the development of the optic lobes. I should have said optic vesicles. 



