BALFOUR'S THEORY 



123 



bolic rods and cones were to have an adequate blood supply (the chori- 

 oid) without this lying between them and the light and blurring the 

 image. Moreover, it must be remembered that we have no certainty 

 whatever that the chordate nervous system originated as a tube — the 

 lowest vertebrates, which should show the most primitive situation, de- 

 velop it as a solid cord and canalize it secondarily. 



Fig. 49 — The foveolce opticae in relation to Balfour's theory. From Walls. 



a, unclosed brain region of neural tube of frog embryo, showing the foveolce optica, /-/, 

 as patches of pigmented columnar cells (after Franz). 



b, c, d, stages in the evolution of the eyes, based on the development of the foveolae into 

 the retincB (after Lange). 



Fig. 50 — Illustrating Schimkewitsch's 



version o 



f Balfour's theory. 



(Deriving the lateral eyes from one of several pairs of photosensory pits in the surface ecto- 

 derm, of which the foveolaa optica are the embryological counterparts). From Walls, after 

 Schimkewitsch. 



