1887.] on the Pineal Eye in Lizards. 25 



running forwards from the roof of the brain within the skull till 

 it reaches the parietal foramen ; but the distal vesicle, transformed 

 into an eye, has become completely separated off from it. This 

 again may be taken as an example of a large number of lizards, 

 including such forms as Varanus hengalensis, Anguis fragUis, Seps 

 chalciclica and Calotes opMomaca. In other forms, still further de- 

 generation seems to have taken place, or, to speak more correctly, 

 the epijDhysis appears to develop to a certain point, but never, at 

 the present time, to reach the stage at which it becomes transformed 

 into an eye. If the brain of Cydodiis gigas be examined, the epiphysis 

 when cut in section has merely the form of a hollow dorsal process 

 running forward from the brain till it reaches the parietal foramen ; 

 within this it expands to form a vesicle, but apparently development 

 stops at this, which must necessarily be a stage passed through in 

 the formation of every pineal eye. The pineal stalk never becomes 

 solid, nor do the walls of the vesicular enlargement become modified 

 into lens and retina, though there is a slight difference to be noticed 

 between the anterior and posterior walls, indicating perhaps the 

 earliest of the series of changes, whereby out of the former is pro- 

 duced the lens, and out of the latter the rods and external-lying 

 elements of the retina. 



These three examples — Hatferia, Varamis, and Cijclodus — will 

 serve to illustrate the more important stages of development met with 

 at the present time in the epiphysis of lizards. 



We may now turn for a minute or two to the consideration of the 

 question, whether it is possible to connect this single median eye with 

 any structure in lower forms, whether the latter, in fact, possess any 

 organ out of which we may suppose the epiphysis of higher forms to 

 have been gradually evolved. Amongst the lowest forms of Vertebrata 

 known to us— the Tunicata — we find that at one stage of their de- 

 velopment they possess a dorsal nerve-cord, whose anterior end 

 becomes swollen out and thus forms what we may call a brain ; in 

 fact this homology with the same part in higher forms is further 

 strengthened by its relationshij) to tlie anterior end of the notochord 

 which, running the whole length beneath the j)osterior part of the 

 nerve-cord, stops just before reaching this anterior swollen extremity. 

 In the adult of most forms of Tunicata the latter is the only j)ersistent 

 portion of the whole nervous system, and its development alone 

 enables us to homologise it with the brain of such an animal as a 

 lizard. Now if this Tunicate brain be examined it is found to possess 

 a single eye placed not quite but nearly in the median line, hut com- 

 pletely ivithin the brain cavity. The Tunicata are, individually, small 

 and transparent, and thus the light can as easily affect an internal as 

 an externally-placed eye. The question naturally arises, suj^posing 

 the Tunicata to have developed from some ancestor common to them 

 and the higher Vertebrata, has this single eye anything to do with 

 the pineal eye ? The latter cannot be directly developed out of the 



