140 ARTHUR DENDY. 



overlies the much smaller left vesicle, a conclusion which 

 appears to be supported by the observations of Studnicka (35) 

 on the development of Petromyzon planeri. The latter 

 author, however, does not appear to realise the paired character 

 of the two vesicles, which he terms " pineal " and '' para- 

 pineal " respectively ; the former, which he supposes to be of 

 greater antiquity, apparently corresponding to the right, and 

 the latter, which he supposes to be of more recent origin, to 

 the left parietal eye. Why these two organs should so per- 

 sistently tend to alter their positions from the primitive trans- 

 verse to the sagittal plane is a mystery which cannot at present 

 be explained. 



Before leaving the question of the paired origin of the parietal 

 eye, I should recall the very interesting and significant fact 

 that Locy (21) considers the epiphysis of Elasmobranchs to be 

 formed from a united pair of accessory optic vesicles ; and that 

 he has recorded (23) the discovery of accessory optic vesicles 

 in the chick, and put forward the view that the Vertebrate 

 eyes are segmental. 



(e) The Relations between the Parietal Stalk, the 

 Epiphysis, and the Brain. 



The organ which I term in this paper the " parietal stalk,'* 

 as I have already pointed out, is commonly, if not invariably, 

 identified by writers on the subject as a portion, at any rate, 

 of the epiphysis cerebri or pineal gland. By those who 

 hold that the parietal eye is formed at the expense of the 

 stalk, the former is commonly regarded as the distal, and the 

 latter as the proximal part of the epiphysis. 



In my summary of results I announced my belief that the 

 parietal stalk does not represent the epiphysis, basing this con- 

 clusion upon a more or less mistaken identification of the early 

 and strongly developed superior commissure which arises just 

 in front of the stalk as the posterior commissure.^ Although my 

 conclusion was thus originally founded upon a mistake, and 



' I still believe that the superior commissure may form the anterior part of 

 the posterior commissure of the adult. 



