PARIETAL EYE AND ADJACENT ORGANS IN SPHENODON. 117 



At Stage N, however, when the parietal eye and stalk are 

 both already clearly differentiated, the paraphysis appears as a 

 small, broadly rounded diverticulum of the roof of the brain, 

 close to the junction of the prosencephalon with the thalam- 

 encephalon. Owing partly to a not very well marked depres- 

 sion! of the roof of the brain just behind it, this diverticulum 

 already points backwards, as shown in fig. 6, towards the 

 parietal eye, from which it is separated by almost the entire 

 length of the roof of the thalamencephalon. 



The cavity of the paraphysis on its first appearance commu- 

 nicates by a wide aperture with that of the fore-brain, and its 

 wall is composed of a layer of columnar cells similar to those 

 forming the wall of the brain in its immediate neighbourhood. 

 It is interesting to note that already a distinct blood-vessel is 

 conspicuous in sections just behind the paraphysis, lying on the 

 roof of the thalamencephalon (fig. 6). 



Stage O. 



The Parietal Eye and Stalk. — At Stage O, comparable 

 with a chick of about five and a half days, the parietal eye and 

 its stalk are conspicuous externally (as, indeed, they were 

 already at Stage N) when the head is examined as an opaque 

 object without staining. It will be seen, from an inspection of 

 fig. 12, that there is on top of the head a dark-looking, elon- 

 gated, quadrangular area,^ tapering behind to an acute angle 

 which lies just in front of and between the optic lobes, while 

 anteriorly it ends in a less acute angle just behind and 

 between the cerebral hemispheres. 



In the posterior angle of this area, just behind the paired 

 eyes and in front of the optic lobes, are visible two round spots 

 of an opaque white appearance. One of these lies just in front 

 of and slightly above the other, and also slightly to the left. 



' This depression, or slight fold of the brain-roof, appears to mark tlie 

 junction between prosencephalon and thalamencephalon, so that the para- 

 physis is to be regarded as an outgrowth of the hinder part of the prosen- 

 cephalon. 



^ This area belongs chiefly to the thalamencephalon, but its most anterior 

 portion, containing the paraphysis, belongs to the prosencephalon. 



