THE PINEAL GLAND 299 



differentiation into lens and retina, and in Cyclodus degenera- 

 tion appears to have progressed until the "pineal eye" has either 

 disappeared altogether or perhaps never becomes separated 

 from the remainder of the "epiphysis." 



Spencer, as is well known, regarded the " epiphysis " of 

 the higher chordates as the homologue of the unpaired eye 

 of the larval tunicate, and the " pineal eye " as a secondary 

 differentiation of the distal part of the " epiphysis," with which 

 it may remain connected by an optic nerve formed by a 

 constricted portion of the latter. 



According to more recent investigations — namely, those of 

 Strahl and Martin and Beranek — the nerve of the " pineal eye " 

 really arises quite independently of the " epiphysis," and is con- 

 nected with the roof of the thalamencephalon in front of the 

 latter and in the neighbourhood of the habenular commissure. 

 De Klinckowstrom in 1893 also described the nerve of the "pineal 

 eye " in embryos of Iguana, and traced it to the right habenular 

 ganglion — a very remarkable result. According to him there 

 is sometimes another nerve also which joins the " pineal eye " 

 with the left habenular ganglion, and in one case he found 

 yet a third nerve joining the " epiphysis " or " stalk " to the 

 posterior commissure. In the adult, he tells us, the eye shows 

 the variation characteristic of vestigial organs, and the nerve 

 degenerates. It is obvious that further investigations on the 

 nerve supply of the organs in question are much to be desired. 



The development of the pineal organs in the Lacertilia is 

 extremely interesting. All observers are agreed that at a very 

 early stage two pineal organs appear on the roof of the thalam- 

 encephalon, just in front of the posterior commissure, very 

 much as in fishes, and that one of these, situated in this case 

 more anteriorly, develops into the pineal or parietal " eye," 

 while the other becomes the so-called " stalk " or " posterior 

 epiphysis" (see fig. 1). There is much difference of opinion, 

 however, as to the exact relationships which these two structures 

 bear to one another and to the brain. According to Leydig, for 

 example, they are independent outgrowths of the brain-roof 

 arising one behind the other, and this author also maintains that 

 accessory parietal organs may be developed {e:g. in Anguis). 

 According to another school, of which de Klinckowstrom may 

 be taken as representative, the two organs arise one behind 

 the other but the anterior is really an outgrowth of the 



