768 



is no differentiation of its walls into retina and lens. There is no 

 pigment in any part of it. The dorsal wall is considerably thicker 

 than the ventral, the former being several cells deep, while the latter 

 is only one cell deep, and the cells are not well defined, there being 

 considerable intervals in which none can be seen. The thickest portion 

 of the dorsal wall is in the middle of the dorsal side, but toward 

 the posterior portion of it, and on the inner surface of this portion 

 are two deep pits. Occupying about a third of the cavity is a mass 

 the nature of which is somewhat doubtful; but it appears like a co- 

 agulum in which some cells are embedded. 



All these conditions are shown in the figure. The inner layer 

 of cells of the antero-dorsal wall are short-columnar and are some- 

 what more deeply stained than those of any other portion. A few 

 of the cells of the thinner ventral wall are also somewhat columnar. 



The papers to which I have referred are those of Leydig ('90), 

 Beraneck ('92), Eycleshymer ('92), Klinckowström ('93), Stud- 

 nicka ('93 a and b), Prenant ('93), Lacy ('93), and Francotte ('94). 



The question is inevitable: Is this parapineal organ present in a 

 single individual of Phrynosoma, an atavistic representative of a second 

 pineal eye ? There is certainly much to be gathered from these several 

 papers that suggests an affirmative answer. 



It is the aggregate of evidence, to which all, with perhaps a single 

 exception, contribute something that gives weight to the suggestion. 

 But that furnished by Lacy's paper is probably most important of 

 all ; certainly it would be should his discovery be fully confirmed both 

 for Selacheans and for other vertebrates. This author finds in em- 

 bryos of Squalus acanthias that at a time when the anlages of the 

 paired eyes are mere cups on the cephalic plate, there are two other 

 pairs of quite similar cups immediately behind these. He regards 

 them as accessory optic vesicles. He traces the fate of these as 

 development proceeds, and finds that the first pair forms the wall of 

 the thalamencephalom, from the roof of which the epiphysis is derived. 

 The epiphysis he therefore regards as taking its origin from paired 

 optic vesicles which are homologous with those giving origin to the 

 lateral eyes. ' 



The second accessory pair is wholly lost by degeneration at an 

 early developmental stage. 



Let it be fully established that the ancestral vertebrate possessed 

 two or more pairs of severally homologous eyes situated on the dorsal 

 side of the central nerve axis, and we should have a good starting 

 point for an explanation, not only of the epiphysis and pineal eye, 



