126 ARTHUR DENDY. 



into an outer thinner and an inner thicker layer, as shown in 

 Spencer's figures (33). 



Towards the end of Stage R, as already noted, an important 

 change has taken place in the relative positions of the parietal 

 eye and stalk. The eye no longer lies over the end of the 

 stalk, but appears to have shifted forwards, so that it lies 

 slightly in front of the middle of the thalamencephalon and 

 over the tubules of the paraphysis, being separated from the 

 end of the parietal stalk by a wide interval. The relative 

 positions of the parts under discussion at this period are shown 

 in fig. 15, representing a longitudinal vertical section through 

 the thalamencephalon in the median plane. With the excep- 

 tion of the forward shifting of the eye the relations are much 

 the same as in the earlier part of Stage R. 



As regards the parietal eye itself, the changes are not very 

 great as compared with younger embryos of this stage. The 

 optic cup, however, is deeper than before, and its axis is in- 

 clined backwards and downwards (fig, 16). The retina is still 

 clearly differentiated into two layers, as shown in figs, 16 and 

 17, and as described in the younger embryo. The black pig- 

 ment (Pig.) is more abundant, but is still almost confined to 

 the inner half of the inner layer of the wall. 



In both the earlier and later periods of Stage R a consider- 

 able amount of coagulated humour is present in the cavity of 

 the parietal eye, chiefly adhering to the inner surface of the 

 retina, as shown in fig. 17 {Coag.), with occasional nuclei, as 

 shown in fig. 18. It is seen also in much smaller quantity 

 adhering to the inner surface of the lens. In figs. 14, 15, and 

 16 it is omitted for the sake of clearness. 



The Nerve of the Parietal Eye. — By far the most 

 important change which marks the later part of Stage R is 

 the appearance of the nerve of the parietal eye. This is seen 

 in longitudinal vertical sections as a faintly staining, deli- 

 cately fibrillated band attached somewhat posteriorly to the 

 outer layer of the retina, as shown in fig. 16 {Pa. N.). From 

 its point of attachment it runs backwards and downwards be- 

 tween the distal part of the parietal stalk and the tubules of 



