120 



THE GENESIS OF THE VERTEBRATE EYE 



The big difficulties which an eye-origin theory must hurdle are: (a) 

 the inversion of the retina — the fact that the vertebrate visual cells point 

 away from the light; (b) the nature of the visual cells before they be- 

 come photosensory, and the question of their location at the time they 

 did so; and (c) the question of the status of the lens before it became 

 associated with the retina as a dioptric structure. 



i^"^#il -*- inf 



Fig. 45 — Sagittal section of 'brain' of Amphioxus. 



(In the position it normally has in the living animal in its burrow). From Walls, after Franz. 



aps- anterior pigment spot; dc- two of the dorsal cells of Joseph; inf- infundibular organ, 

 whose photosensory elements are flagellated ependymal cells. 



Early Theories — Between 1874 and 1929 a series of investigators saw 

 the beginnings of the vertebrate eye in the anterior pigment ^ot of 

 Amphioxus (Fig. 45, aps). Even by 1890, however, experiments had 

 indicated that this 'eye' is not sensory at all, and at the present time 

 this is considered certain. 



