124 ARTHUR DENDY. 



oval nuclei, between wliichj on the inner aspect^ the boun- 

 daries of columnar cells are faintly discernible. In one late 

 embryo of this stage I have observed the wall of the parietal 

 stalk to be clearly differentiated into two layers (fig. 21), an 

 outer thinner one composed of a single layer of short columnar 

 cells, and an inner very much thicker one, so densely crowded 

 with large oval nuclei arranged in many but irregular tiers, 

 that it is impossible to make out the cell boundaries satis- 

 factorily. 



Towards the end of Stage R a patch of black pigment is 

 deposited in the antero-ventral wall of the parietal stalk, near 

 its distal extremity (figs. 15, 16, 21). This pigment resembles 

 that of the parietal eye, having the form of small black 

 granules lying between the innermost cells of the wall, next to 

 the lumen. It ajjpears to be constant, for I have observed it 

 both in transverse and longitudinal sections of embryos at 

 this stage. De Klinckowstrom (19) has observed pigment in 

 the same situation in Iguana at a certain stage of develop- 

 ment, but apparently disappearing later. In Sphenodon 

 also it would seem to disappear in the adult, for none is shown 

 in Spencer^s figure (33). Ritter also (29) in Phrynosoma 

 coronata has observed pigment in what he terms the 

 '' epiphysial vesicle,'^ which appears to be homologous with 

 the distal extremity of the parietal stalk in Sphenodon. 



The Parietal Eye. — At the commencement of Stage R 

 we see the parietal eye in sagittal section lying over the 

 hinder part of the thalamencephalon, and above the flattened 

 distal extremity of the parietal stalk (fig. 14). The end of 

 the stalk lies wedged in between the eye and the brain, and 

 between the stalk and the eye there is a very thin layer of 

 connective-tissue cells. Above the eye, between it and the 

 superficial epidermis, is a rather thick layer of connective 

 tissue, forming part of the dermis, which, however, is here 

 thinner but denser than elsewhere. The upper surface 

 of the lens is pressed close against the under surface of the 

 dermis. 



In the eye itself important changes have taken place. The 



