132 



THE GENESIS OF THE VERTEBRATE EYE 



to concentrate light upon the photosensitive lining of the diencephalon. 

 Its locus somehow escaped involution with the neural tube and later 

 moved laterally to be taken over by the new retina (Fig. 59) . No onto- 

 genetic conditions support this idea, and like the placode theory it stands 

 or falls with the demonstrability of a self-differentiating lens anlage. 



Fig. 59 — Illustrating Franz's theory. From Walls, after Franz. 



a, ancestral surface eye corresponding to the infundibular organ of Amphioxus prior to the 

 closure of the neural tube, b, later stage corresponding to the foveola; opticEe, with the 

 future lens-forming area labelled la. c, stage of general photosensitivity of lining of dien- 

 cephalon. The lens (shown in an earlier stage on the left, a later one on the right) is 

 evolving just outside the region of involution, d, stage of appearance of dorsal diencephalic 

 evagination — the future pineal eye; the lentogenic areas have shifted still farther laterally. 

 e, final condition of the pineal (p) and lateral eyes (/e); the lens is now embryologically 

 derived from the skin far distant from its original location. 



The experimental morphologists are very fond indeed of doing things 

 to embryonic eyes to see what they will do in return. Someday, their 

 juggleries may disclose that in some species of fish or amphibian a lens 

 will start to develop without the presence of an optic vesicle. Until 



