THE PINEAL BODY 41 



a pigmented structure and in it-appears a number of nerve fibers. 

 In the later embryonic stages the stalk connecting the end- 

 vesicle of the parapineal organ with the brain attenuates, loses 

 its lumen, and shows the presence in it of numerous nerve fibers 

 which may be traced to the commissura habenularis. The rapid 

 elongation of the stalk in the parapineal and pineal organs as 

 development advances causes these structures to be moved 

 further away from the roof-plate and near the under surface of 

 the skull. The general direction of this growth is cephalodorsad. 

 Gaskell 145 showed in Ammoccetes a right and left pineal eye. 

 It is his opinion that the pineal and parapineal organs represent 

 a paired set of eyes. Their relation to each other, in which the 

 parapineal organ occupies the more cephalic position, was deter- 

 mined, according to Gaskell, by the exigencies of development. 

 In reality, however, he believes that the ancestors of vertebrates 

 must have possessed a pair of median eyes. 



Dendy 86 also observed in cyclostomes a double evagination 

 from the roof-plate giving rise to the epiphyseal complex. It is 

 his opinion that the right evagination produces the parietal eye 

 while the left becomes the parapineal organ, and Dendy, like 

 Gaskell, maintains that the ancestors of the vertebrates must 

 have been possessed of a pair of parietal eyes which may have 

 been serially homologous with the ordinary vertebrate eyes. 

 Scott ('81) 349 and Dohrn (75) 95 both showed that the epiphyseal 

 complex developed as evaginations from the roof of the in- 

 terbrain. These observations were essentially confirmed by 

 Shipley ('87), 354 Owsiannikow ('88), 295 Studnicka ('93), 384 and 

 Kupffer C94). 224 



2. The development of the epiphyseal complex in selachians 



Balfour 10 in 1878, in Acanthias, d'Erchia 109 in 1896, in Pris- 

 tiurus, and Minot 277 in 1902, also in Pristiurus, investigated the 

 development of the epiphyseal complex. According to all of 

 these authors, a single evagination arises in the roof-plate im- 

 mediately in front of what is later to be the posterior commissure. 

 This evagination gives rise to the pineal organ inasmuch as the 

 parapineal organ does not appear in selachians. From its 



