DEVELOPMENT OF THE NERVOUS SYSTEM 523 



paraphysis, but the difference in time of their origin is very 

 sHght. 



In forms on either side of the m-odela we have the epiphysis 

 usually double. It has never been considered double in urodela, 

 to the writer's knowledge, except by Cameron ('03). I believe 

 that in my description I have shown that it is fundamentally 

 double in this form. The two epiphysial outpocketings are closely 

 associated, both in position and time of origin. The posterior 

 vesicle, however, antedates the anterior one slightly in two respects : 

 the solid outgrowth of the former appears earlier than in the latter ; 

 the cavity of the posterior outgrowth appears earlier than that of 

 the anterior one. That they later have a common opening and 

 that they never become separated is nothing fundamentally 

 against their being two morphological structures. In amphibia 

 the epiphysis is more rudimentary than in pisces or reptilia. This 

 might account for the two vesicles remaining together. If the 

 epiphysis of this form had reached a greater development, I think 

 that we logically could have expected the anterior vesicle in the 

 later embryonic stages to have become constricted from the 

 posterior vesicle. 



The posterior commissure first forms near the epiphysis in the 

 constriction which separates the diencephalon from the mesen- 

 cephalon. By the growth of that portion of the roof lying between 

 the posterior commissure and the epiphysis, the two structures 

 become separated. They again approach each other due to their 

 extension and because the roof of the diencephalon assumes a 

 different direction in its growth, thickening instead of lengthening. 



Summary 



The epiphysis of Desmognathus fusca is morphologically 

 double. 



The posterior outgrowth forms first; it is followed by an anterior 

 evagination which forms independently. 



As growth proceeds they come to have a common ostium into 

 the third ventricle. 



The epiphysis is formed on the second diencephalic segment. 



