PAKIETAL EYE AND ADJACKNT ORGANS IN SPHENODON. 119 



of the lens. Here and there a clear-looking space appears be- 

 tween the elongated cells, containing what seems to be a much 

 smaller nucleus. 



In the cavities of both stalk and eye the usual coagulum is 

 present with an occasional small nucleus. These nuclei pre- 

 sent a curiously lobed or agglomerated appearance, and appear 

 to me to be thrown off from the surrounding walls (figs. 19, 

 20, Nu.). In fig. 19 one appears to be just leaving the inner 

 surface of the lens. They are possibly identical with the 

 smaller nuclei noticed above in the interior of the lens, and 

 which also occur in the ventral wall of the eye, and perhaps in 

 the wall of the stalk, but I have not sufficient evidence to 

 decide this point. Karyokinesis is going on both in the wall 

 of the stalk and of the eye. 



Just behind the parietal stalk the roof of the brain has 

 already become differentiated into two layers (fig. 10), the 

 outer finely punctate in longitudinal section and almost without 

 nuclei, the inner with numerous large oval nuclei. 



The Paraphysis. — At this stage the paraphysis has elon- 

 gated backwards, and its wall has begun to be irregularly folded 

 (fig. 11). Its cavity still communicates freely with that of the 

 brain, and the blood-vessel is still conspicuous just behind its 

 apex. 



I have already pointed out in dealing with the parietal eye 

 and stalk the relative positions of these organs and of the 

 paraphysis, which are readily seen on examining the upper 

 surface of the head of embryos at this stage (fig. 12). 



Stage P. 

 Of Stage P I have only a single specimen, and it is so 

 similar in external characters to Stage O that I have not 

 thought it necessary to cut sections at present, especially as 

 its histological condition is doubtful. 



Stage Q. 

 Of Stage Q I have also only a single specimen, which 

 was dead when it reached me, and useless for histological 

 purposes. 



