PARIETAL EYE AND ADJACENT ORGANS TN SPHENODON. 135 



not to the " pineal gland/' but to the parietal eye only. This 

 is exactly my view with regard to this organ in Sphenodoii, 

 in which it appears that the ^^ stalk '^ or '^ pineal gland '^ or 

 "epiphysis'' (so called) develops a little later than the parietal 

 eye itself. The eye, it will be remembered^ first appears as a 

 small round vesicle, which I have termed the " primary pari- 

 etal vesicle," budded out on the left side of the middle 

 line. This vesicle^ according to my interpretation, soon 

 becomes completely closed, and a second vesicle appears as an 

 outgrowth of the roof of the fore-brain, beneath and slightly 

 to the right of the primary vesicle, pushing the primary 

 vesicle forwards. This second vesicle forms the "parietal 

 stalk." For a long time its cavity retains free communica- 

 tion with the cavity of the fore-brain, but at Stage R it has 

 become completely shut off from the latter, a fact 

 which will be seen to have considerable importance when we 

 come to consider the relation which the parietal stalk bears to 

 the " epiphysis " of the adult. 



{d) Evidence of the Paired Origin of the Parietal 

 Eye and Stalk. 



The evidence in favour of the originally paired character of 

 the parietal eye in Sphenodon is derived principally from 

 the fact that the eye itself arises on the left-hand side of the 

 middle line, while the "parietal stalk" appears almost or quite 

 in the middle line, and therefore a little to the right of the 

 parietal eye. These relations appear to be very constant. 

 Thus I have observed the first origin of the primary parietal 

 vesicle (= parietal eye) to the left of the middle line in the 

 only two embryos of which preparations suitable for demon- 

 strating this point have been made (vide figs. 2, 5) ; and later 

 on, when the parts in question have become conspicuous 

 externally, I have observed the young parietal eye lying to 

 the left of the stalk (fig. 12) in nearly every embryo examined, 

 though by the time Stage R is reached, as already noticed, it 

 appears to have become median (fig. 13). 



