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 Reptiles. 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 Reptiles and the Slow-worms, we 
find the upper wall may in many cases become thickened to 
form a transparent epithelial plate (7”., Fig. 85), which is often 
lens-shaped, while the rest of the epiphysial vesicle (7’.), 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 (Béraneck). The organ is invested by a capsule of 
connective tissue (cp.). 
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.’ 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, Reptiles, 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 Klinckowstrém 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., Anat. Abth. Bd. v. p. 177.] 
