HISTORY OF THE MEDIAN EYES 



339 



sub-median eyes of the lamprey, the 'pineal' and 'parietal', thus seem 

 each to represent one member of an original pair (Fig. 54, p. 126). In 

 the same way, the one eye of the ascidian tadpole (see p. 121) is situated 

 off the mid-line and seems to have a mate in the form of a vestigial mass 

 of tissue on the other side of the head (Fig. 48d, p. 122). 



Neither of the median eyes of a lamprey is built well enough to have 

 images, or anything more than the ability to record the intensity and 

 perhaps the direction of light. In vertebrates higher than the lampreys, 



--EPIDERMIS-^ 

 """■-CORIUM -'- 



SUPERIOR 



HABENULAR 



COMMISSURE 



PARIETAL NERVE 



NEAL TRACT 

 DBRAIN 

 POSTERIOR COMMISSURE 



NEAL TRACT 



IB ANURANl 



DORSAL SAC 



SUPERIOR HABENULAR/ 



COMMISSURE 



POSTERIORI 

 COMMISSURE 



Ic. reptile] 



ID mammal] 



Fig. 124 — Condition of the pineal and parietal (parapineal) eyes in various vertebrates. 

 After Neal and Rand. 



only one of these eyes is ever to be found. The stegocephalians must 

 have had the pineal eye at the height of its development (Fig. 61b, p, 

 p. 137), if we can judge from the size of the foramen for its nerve in the 

 stegocephalian skull. But in modern amphibians it is vestigial, in the frog 

 a mere cyst underlying the skin of the 'brow spot'. In birds it has gone 

 completely, and in mammals it has been converted into the 'pineal gland' 

 of dubious, possibly endocrine, function. 



The parietal eye must have been somehow represented in the stego- 

 cephalians; for, though it is completely lacking in modern fishes and 

 amphibians, it is present as the sole median eye in modern reptiles. It is 



