134 THE STRUCTURE OF MAN 



and to have undergone degeneration in the course of Phylogeny, 

 as the roof of the skull became more and more solid. The nerve 

 belonging to it is, so far as is known, most fully retained in 

 certain Eeptiles. In some animals this organ only occurs in the 

 embryo, and altogether disappears at a later stage. 



In examining the finer histological structure of the pineal or 

 parietal organ in the Lizard-like Eeptiles and the Slow- worms, we 

 find the upper wall may in many cases become thickened to 

 form a transparent epithelial plate (?*"., Fig. 85), which is often 

 lens-shaped, while the rest of the epiphysial vesicle (?*'.), which is 

 often flattened, is differentiated into a multilaminar "retina." 

 " Lens " and " retina " thus arise in complete continuity out of one 

 and the same structure ; and it is only at a late stage in develop- 

 ment that a more or less distinct demarcation between them is 

 effected (Be"raneck). The organ is invested by a capsule of 

 connective tissue (cjp.). 



In many cases the skin which overlies the parietal organ, as 

 well as the connective and dural tissues below it, remain free from 

 pigment, indeed they are sometimes so clear and transparent that 

 they might be considered as a kind of cornea. This justifies the 

 assumption that the function of the organ may not be altogether 

 lost even now. 1 Owsiannikow claims to have found traces of a 

 vitreous body within it. 



According to Leydig, Selenka, and others, there is found in 

 the embryos of various Vertebrates (Selachians, Eeptiles, Mar- 

 supials, and probably in others) another unpaired dorsal appendage 

 of the fore-brain, for which Selenka has suggested the name 

 " frontal organ " or " paraphysis." 



Whereas the epiphysis grows forward, the paraphysis, which 

 arises much later ontogenetically, grows backward and, when the 

 epiphysis is once fixed in the epidermis, pushes itself in under 

 that organ, so that the parietal eye comes to rest on the para- 

 physis as on a cushion. Until the embryo is mature, the 

 epithelial tube of the paraphysis remains hollow and in open 

 communication with the cavity of the brain. 



If it be established that the pineal organ and gland are 

 really sui generis, distinct in origin, there is evidence of three out- 



1 [In view of the intimate relationship between birds and reptiles, it is an 

 interesting circumstance that Klinckowstrom has discovered in embryos of certain 

 of the former (Anser. Larus.) a "brow spot," which in its structural differentiation 

 suggests not only the last trace of a pineal organ, but a pineal scale like that of 

 living lizards. Spengel's Zoolog. Jahrb., Anaf. AUh. Bd. v. p. 177.] 



