DEVELOPMENT OF THE NERVOUS SYSTEM 521 



Discussion 



The literature on the embryology of the pineal region treats the 

 subject, in general, from two points of view: (1) the development 

 of the structures, considering them intrinsically, and (2) their 

 significance in determining the segmentation of this region. For a 

 more extensive summary of the literature than is here given, 

 consult Gaupp ('97) and Warren ('05, '11). 



In the forms below amphibia two epiphysial evaginations have 

 been usually described. Hill ('91), in Coregonus and ('94) in 

 Salmo, Catostomus, Stizotedion, and Lipomis, describes two evag- 

 inations, the anterior one becoming constricted off and lying 

 between the thalamencephalon and mesencephalon and never 

 reaching an eye-like structure. He, however, homologizes the 

 anterior vesicle to the lacertilian epiphysis. In Petromyzon, 

 Studnicka ('95) describes two outgrowths from the roof of the 

 thalamencephalon and homologizes the anterior one (parapineal 

 organ) with the anterior vesicle of Hill. In Amia, Kingsbury 

 ('97) states that the anterior vesicle persists and has a fiber con- 

 jiection with the left habena. Eycleshymer and Davis ('97) 

 also worked on this form and my figures correspond closely to 

 theirs. ]\Iy figure 1 gives much the same appearance as their 

 figure 1, with the exception that in my figure an active cell migra- 

 tion dorso-caudally is suggested in the posterior vesicle. In their 

 figure of a 5 to 6 day embryo (fig. 3) the posterior vesicle has a 

 lumen and the anterior epiphysis is a mass of cells with no lumen 

 as yet. Compare my figure 2 with this. My figure 3 corresponds 

 to their figure 8. Their figures, thus, up to this stage, correspond 

 closely to mine; from this stage on, however, as shown by their 

 drawings, the anterior vesicle becomes constricted off, looses its 

 cavity and shows marks of degeneration. In Desmognathus both 

 vesicles remain, and up to about a 17 mm. stage have a common 

 opening into the III ventricle. Terry ('10) in Opsanus describes 

 two outgrowths, the anterior one being solid. Locy ('93) in Acan- 

 thias finds a double epiphysis and traces its origin to a pair of 

 accessor}' optic vesicles in the medullary plate stage. If this 

 observation prove to be correct it explains phylogenetically the 



