THE PINEAL BODY 219 



served in the stalks; those coming from the pineal end-vesicle 

 terminate in the posterior commissure, while those seemingly in 

 connection with the parapineal end-vesicle end in the habenular 

 commissure. Some collateral evidence is afforded by the appear- 

 ance of a parietal cornea, a fiberless tissue which surrounds the 

 pineal and parapineal end-vesicles. 



All of these histological facts, based upon the observation of 

 cyclostomes, indicate what may be considered an abortive yet 

 a well-advanced attempt to the formation of two eyes. There 

 is no evidence of glandular formation in any part of the epi- 

 physeal complex in cyclostomes. 



Histological evidence in selachians. The characteristics of finer 

 structures, so conspicuous in petromyzon and its congeners, is 

 strikingly absent in the next higher order, the selachians. In 

 consequence of the apparent lack of differentiation, the entire 

 parapineal organ is absent, while the pineal organ, although 

 conspicuous for its size, shows no tendency toward the formation 

 of a retina, pellucida, white pigment, or nerve fibers. It is a 

 question whether the pineal organ in selachians should be con- 

 sidered as a primitive organ or as one in a stage of retrogression. 

 The walls of the end-vesicle are made up exclusively of epen- 

 dymal cells and contain neither spindle nor rod cells. In one 

 form, Scyllium, Galeotti 140 described a peculiar appearance of 

 the cells of the end-vesicle which seemed to indicate a secretory 

 function. This conclusion of Galeotti's depends on the appear- 

 ance of fuchsinophile granules not only in the nuclei of the cells, 

 but scattered diffusely throughout the cytoplasm. Studnicka 391 

 also recognized these cells and, while he was unwilling to attrib- 

 ute any definite function to them, he was of the opinion that they 

 could not be secretory in their nature. 



It is apparent, therefore, in passing from the cyclostomes to 

 the selachians that there is a striking absence of any visual 

 differentiation or any tendency in this direction, while the 

 presence of certain histological characters in the cells furnishes 

 evidence pointing to a possible glandular formation in the end- 

 vesicle of the pineal organ. 



