THE VERTEBRATE EYE 



The origin of the essential sensory cells, the rods and cones, has 

 long remained a matter of dispute. Ever since the time of Scliwalbe 

 (1874) they had been generally considered as neuro-epithelium. 

 Kraiise (1875), however, originally put forward the suggestion that they 



Figs. 248 to 254. — Hypothetical Scheme fob the Phylogenetic 

 Development of Vertebrate Eyes. 



243 



Fig. 248. 



Fig. 252 



Fig. 249. 



Fig. 253. 



Fig. 250. 



Fig. 251. 



Fig. 254. 



Fig. 248. — Photosensitive ciliated ectoderm on the dorsal aspect. 



Figs. 249 and 250. — Invagination of the surface ectoderm to form the 

 neural tube, carrying with it the photosensitive ectoderm. 



Fig. 251. — The formation of the neural tube enclosing the photosensitive 

 epithelium as ependyma. 



Fig. 252. — Commencing evagination of the neural tube. 



Fig. 253. — The formation of one median and two lateral optic vesicles. 



Fig. 254. — Invagination of the surface ectoderm with secondary in- 

 vagination of the lateral optic vesicles to form two lateral eyes with inverted 

 retinse. The surface epithelium takes no part in the development of the 

 median eye which forms its own dioptric apparatus (lens) in the distal part 

 of the vesicle which itself does not undergo secondary invagination and thus 

 forms a verted retina. 



were derived from the ciliated ependymal cells lining the neural tube, 

 the cilia eventually forming the outer segments of the visual cells — 

 a view, however, which he quickly withdrew (1876). The vast 

 authority of these two pioneers in the histology of the visual organs 

 long remained unchallenged, but t he view that this layer of cells might 



