116 ARTHUR DENDY. 



parts, an anterior one forming the parietal eye and a posterior 

 one forming the stalk (or the eye may have budded out from 

 the stalk, which comes to much the same thing). This 

 appears to be the view generally adopted. 



(2) It appears to me more likely that the parietal eye is 

 formed from the primary parietal vesicle, and that the parietal 

 stalk arises as a second, distinct evagination of the roof of the 

 fore-brain after the primary parietal vesicle has become almost 

 if not quite constricted off. This second evagination would 

 appear to take place slightly to the right of the primary parietal 

 vesicle, which seems to be pushed forward in the process. 

 This view of tlie case is strongly supported by the following 

 considerations : — [a) The primary parietal vesicle is already 

 almost constricted off from the brain, and has acquired the 

 characteristic flattened oval form of the young parietal eye 

 before the parietal stalk makes its appearance at all. At this 

 stage (fig. 4) it projects backwards as well as forwards from its 

 attachment to the brain, while the parietal stalk when it 

 appears projects forwards only, {b) The parietal eye, like the 

 primary parietal vesicle, at first lies to the left of the middle 

 line, while the parietal stalk is median, or nearly so. 



The constancy of the left-sided position of the young 

 parietal eye will be discussed later on ; meanwhile I may state 

 that I have observed it in a sufficient number of cases to justify 

 me in attaching considerable importance to it, and I believe it 

 affords almost conclusive evidence in favour of my view that 

 the parietal stalk is a second and independent outgrowth of 

 the roof of the fore-brain. If its cavity ever communi- 

 cates with that of the parietal eye, which is doubtful, it is 

 probably because the second evagination involves the point of 

 attachment of the first to the brain before the opening of the 

 first has closed up. 



The Paraphysis. — This structure does not appear until 

 some time after the appearance of the primary parietal vesicle. 

 It is certainly not present at Stage L, and I have not been 

 able to satisfy myself as to whether it is present or not at 

 Stage M. 



