1887.J on the Lineal Eye in Lizards 23 



vesicle : in the skull wall itself in most lizards there is present a 

 distinctly marked hole, and in this, which is called the parietal foramen 

 lies the small vesicle. Changes take place in the walls of the optic 

 vesicles which transform them into the sensory parts of the paired 

 eyes, whilst the small vesicle formed out of the dorsal outgrowth of 

 the brain, or epiphysis, becomes formed into the single pineal eye. 

 Thus from the walls of the fore brain are developed in lizards three 

 eyes — a single median and two paired ones. 



Digressing for a moment, it is interesting to notice that this is by 

 no means the only example of a single median eye which is known in 

 the animal kingdom. Almost every people has its myth of a median- 

 eyed race of men, who may, as in the Grecian Cyclops, be represented 

 with the single eye, or with this in addition to the paired eyes : it is 

 curious also to find that Buddha and Siva are represented with median 

 eyes : whilst to such an extent is this idea carried, that even the 

 Nautch girl paints on her forehead the outline of a single median eye. 



Leaving the mythical we come to animals in which such a structure 

 is actually present: in the class Crustacea we meet with many 

 examples of this ; first in the little fresh-water Cycloj^s and all its 

 allies, there is only the one solitary eye, the presence of which has 

 gained for it its name. In other Crustacea again we find that 

 the animal on leaving the egg-cascwhas a very definite form, quite 

 unlike that of the adult : it is, amongst other things, always provided 

 with three, and no more or less than three pairs of a2)pendages by 

 means of which it moves about ; it has in addition always a single 

 median eye which, inasmuch as the animal in this stage is called a 

 uauplius, is known as the nauplius eye. As development goes on, 

 there appear two eyes, one at either side of the original one : in some 

 cases the latter may disapj^ear, in others it may be retained, though 

 the lateral paired eyes always become larger than the single one, and 

 thus, though there is no real connection between the two whatever, we 

 find in Crustacea a mCLlian and two paired eyes present just as in 

 lizards. 



Turning to the latter, we find the epiphysis, as before said, 

 more highly developed than in any other living animal. The distal 

 vesicular expansion is present in many forms, but here only is it 

 transformed into an eye and connected with the brain by a solid 

 pineal stalk developed from the proximal part of the epiphysis and 

 serving doubtless as an optic nerve. 



If the head of a lizard, such as an Iguana or Calotes, be examined, 

 there is seen on the middle line dorsally, and somewhat behind the 

 level of the paired eyes, a peculiarly modified scale ; it bears a circular 

 space, which may be raised into a dome-shape, or may be merely 

 surrounded by a raised rim, but is always noticeable by its whiteness, 

 this being due to an absence of pigment in the skin of this par- 

 ticular spot. The modified scale indicates the position of the pineal 

 eye lying beneath, and thus of the parietal foramen enclosing the 

 eye : it does not necessarily follow that no eye is present, because 



