128 AETHUE DENDY. 



lateral ventricles^ between which its opening (0. Par.) is 

 situated. Fig. 24 shows the paraphysis {Par.) cut through 

 above the commissura fornicis. Fig. 35 shows it cut through 

 above the roof of the third ventricle, and below the parietal 

 eye, and fig. 26 shows it cut through just in front of the 

 parietal stalk. 



Accessory Vesicle. — At the commencement of Stage R 

 the tubules of the paraphysis do not extend so far backwards 

 as is represented in fig. 15^ and between them and the parietal 

 stalk there lies a wide space of irregular shape (fig. 14, Ac. V.). 

 This space I propose to term simply an accessory vesicle; 

 its possible homologies will be discussed later on. It is at 

 once distinguished from the paraphysial tubules by the histo- 

 logical characters of its walls, which are composed of much 

 thinner epithelium, though not so thin as those of the blood- 

 vessels. It is further distinguished from the blood-vessels by 

 the absence of corpuscles from its cavity. It gives off a few 

 irregular diverticula or sacculations, but I have not been able 

 to detect any connection between its lumen and the cavity of 

 the third ventricle, nor does it appear to communicate with 

 either the paraphysial tubules or the blood-vessels. Although 

 so conspicuous at the commencement of Stage R, it has 

 completely disappeared in later embryos of this stage, its 

 place being taken by paraphysial tubules (figs. 15, 16). 



Stage S. 



Of this stage, the last before hatching, according to my 

 classification, I have received only dead specimens, useless for 

 histological purposes. 



In the adult Tuatara the parietal eye is no longer recognis- 

 able externally, although still very highly organised ; but, 

 according to Thomas (38), in the recently hatched Tuatara it 

 still shows as a dark spot through the translucent skin over 

 the parietal foramen. 



