126 



THE GENESIS OF THE VERTEBRATE EYE 



offers no account of the lens, it gives as good an explanation of the retina 

 and its inversion as does Balfour's theory; and both hypotheses are widely 

 taught at the present time. Acceptance of either is impossible, however, 

 unless the mode of development of the rods and cones indicates either 

 that they might have been already photosensory while still in the skin. 



Fig. 54 — Illustrating Studnicka's theory. From Walls, after Studnicka. 



The sensory cells of the median and lateral vertebrate eyes are derived from the flagellated 

 ependymal cells which line the neural tube, c represents the larval lamprey, in which the 

 eye is temporarily functional though the retina ('Retina A' — see p. 117) is still only an 

 uninvaginated optic vesicle and the lens is flat and useless. 



or that they might have been derived from the photosensory ganglion 

 cells of Hesse's organs or the similar 'Joseph's cells' in the head region 

 of Amphioxus (Fig. 45, dc). 



Studnicka's Theory — Unfortunately the cytogenesis of the rods and 

 cones supports neither Balfour nor Boveri, but confirms a radically dif- 

 ferent hypothesis first offered in 1912 by Studnicka, and which has yet 

 to be given consideration in any of the various text-books which afford 

 a little space to the eye-origin problem (Fig. 54) . 



