THE PINEAL SYSTEM OF FISHES 203 



behind the optic grooves, gives rise to the walls of the thalamencephalon 

 and the principal outgrowth from it, the epiphysis. When the neural 

 tube is closed the two members of the pair unite to form the epiphysis 

 which in his opinion is therefore double in origin, being formed from a 

 united pair of accessory optic vesicles. He also considered it highly 

 probable that the enlarged distal end of the epiphysis in Squalus is homo- 

 logous with the pineal eye of those forms in which it is differentiated as a 

 median eye. 



With reference to these observations it would be interesting to obtain 

 further observations on these early stages in other animals. The majority 



Fig. 143. 



A : Embryo of Squalus acanthias 3 mm. long ; three somites ; showing the first 

 appearance of the optic vesicle, Op. 



B : Slightly older embryo showing in addition to the primary pair of optic vesicles 

 two pairs of accessory vesicles, Aop.', Aop." 



C : Embryo with open neural groove placed in such a position as to give an 

 external view of the vesicles on one side and an internal view on the opposite 

 side of the neural folds. Locy believed that the accessory vesicles were 

 homologous with the optic vesicles and that the pineal outgrowth was formed 

 in Squalus acanthias by the union of the anterior pair of accessory optic 

 vesicles. 



(Redrawn from Locy's figures : Anat. Ariz., 1893, Bd. IX.) 



of writers describe the pineal outgrowth as being primarily single and 

 first appearing after the neural tube has become closed, the subdivision 

 of the primarily single diverticulum into parietal sense-organ (when this 

 is developed) and epiphysis taking place later. As far as we are aware, 

 the development of two separate diverticula, arising independently of 



