PINEAL EYE 



'■S 



strate its real significance as an eye, with both retina and lens, in 

 the Reptilia. 



It has now been ascertained that there are two organs 

 developed on the roof of the diencephalon (thalameneephalon) ; 

 they take up a median position : the pineal behind, the parapineal 

 or parietal in front. In Petromyzon both are present, growing- 

 out as hollow processes expanding into an e}^e-like vesicle distally, 

 the pineal, however, being more fully differentiated than the para- 

 pineal. The stalk of the former contains nerve-fibres passing to the 

 posterior and habenular commissure, that of the latter fibres going 

 to the habenular commissure (Fig. 14). In all other living Craniata 

 the pineal organ is in a more or less degenerate condition and the 



Fig. 14. 



Longitudinal section of the pineal and parapineal organs of the larva of Petromyzon, enlarged 

 (from the ligures of Studnic'ka). ap.c, anterior dorsal commissure ; ep, epidermis ; h.c, liabenular 

 commissure; h.g, habenular ganglion; m, roof of mid-brain ; », nerve; p, pineal eye; pp, 

 parapineal eye ; pr, paraphysis. 



parapineal is quite vestigial, with the exception of the Reptilia, in 

 which it is occasionally found in a highly developed state, with 

 lens and retina (De Graaf [181], Spencer [412], Beraneck [35a], 

 Studnicka [430]). 



According to Hill and Dendy, there is reason to believe that 

 the dorsal eyes are of paired origin — the parapineal being the left, 

 and the pineal the right, each connected with the habenular 

 ganglion of its own side — a conclusion which is supported by 

 ■embryological evidence (Dendy [112], Cameron [77], Hill [213ft]). 



The epidermis of the Craniates, unlike that of AmpMoxus, 

 is formed of many layers of cells continually being renewed from 

 the lowermost Malpighian stratum. 



The alimentary canal behind the pharynx becomes differentiated 



