PARIETAL EYE AND ADJACENT ORGANS IN SRHENODON. 139 



primary fore-brain of Salmo^ Catostoraus, Stizostedion, 

 LepomiSj and Amia. The anterior of these two is rudi- 

 mentary, and appears to lie constantly to the left of the 

 posterior. He concludes that the anterior epiphysial vesicle 

 is homologous with the parietal eye of Lacertilia, while the 

 posterior, or epiphysis itself, is homologous with the epiphysis 

 of Lacertilia ; and he thinks it probable that in their primitive 

 position the two vesicles were side by side. 



I need hardly point out that my own observations on 

 Sphenodon in the main strongly support the important 

 results arrived at by Hill. Indeed, except that in Coregonus 

 the posterior vesicle appears to arise first, the agreement 

 between the early stages of the types investigated by us is 

 most remarkable ; and I think there can be little doubt that 

 the parietal eye was not originally an unpaired sense-organ, 

 as usually supposed, but one of a pair, of which, in most cases 

 at any rate, the left at the present day has the eye-like 

 structure most fully developed. In bony fishes the right 

 parietal eye is evidently represented by the '^ epiphysis,'' as 

 described by Hill. In Lacertilia it is represented by what I 

 still prefer to term the " parietal stalk,'' which is therefore 

 homologous with the ''epiphysis" of bony fishes; but how far 

 this latter is homologous with the " epiphysis " of Sauropsida 

 and Mammalia as that organ is ordinarily understood is 

 another question. 



The fact that, both in the various types of fishes investigated 

 by Hill and Sphenodon, it is the left vesicle which alone or 

 first separates from the brain, and either degenerates or gives 

 rise to a parietal eye, appears to me very remarkable. It is not, 

 however, by any means certain that it is always the left parietal 

 eye which is best developed. Thus in Lacerta, according to 

 Hoffmann's description (18), the parietal eye would appear 

 to be formed from the right vesicle, and the stalk from the left 

 one. 



Gaskell also (12), who lays great stress upon the paired 

 origin of the parietal eye in the lamprey, believes it to be the 

 right parietal eye which is most perfectly developed, and which 



