132 ARTHUR DENDY. 



observations certainly seem to indicate that such an interpre- 

 tation is incorrect. As already stated, I found at Stage E, a 

 nerve which appears to correspond exactly so far as observed 

 with that described by Beraneck. It passes in front of and 

 beneath the distal extremity of the parietal stalk, and becomes 

 connected with the outer layer of the retina (figs. 15 and 16). 

 Unfortunately I have been unable to trace it to its point of 

 origin from the roof of the brain, but I have little doubt that 

 the connection takes place at or near a point immediately in 

 front of the origin of the parietal stalk, corresponding to what 

 Beraneck terms the parietal centre, and lying between the 

 posterior ends of the two ganglia habenulse, above the fibres of 

 the superior commissure (fig. 26). It seems probable that the 

 nerve is actually connected with one or other of the ganglia 

 habeuulse, as iu Ammocoetes, but I have been unable to detect 

 any such difference in size between the two ganglia as occurs 

 in the lamprey. The fact that the parietal eye itself in 

 Sphenodon originates on the left side of the middle line 

 seems to make it probable that the nerve is connected with 

 the left ganglion habenulse. The observations of de Klinckow- 

 strom, however, point to the right ganglion. Possibly the 

 nerve crosses over from right to left, but this is a question 

 which cannot be decided in the present state of our knowledge, 

 and one which is well worthy of further investigation. We 

 may safely take it as an established fact, however, that the 

 parietal eye has a special nerve which lies in front of and is 

 not derived from the parietal stalk (^'epiphysis''). 



(c) Relations of the Parietal Eye to the Parietal 



Stalk. 



Embryologists who have studied the question appear to be 

 divided into two schools on this subject. The first and older 

 school maintain that the parietal eye is either the distal 

 extremity of tlie stalk ("epiphysis'"') separated from the 

 proximal, or that it is an outgrowth or diverticulum of the 

 stalk. The second and more modern school hold that the 



