SENSE ORGANS. 



One point regarding the eye was formerly emphasized. As 

 will be seen from the method of formation of the eye by invo- 

 lution from the skin (see Fig. 84), the layer of rods and cones 

 is homologous with the superficial layer of the skin, while the 

 ganglionic layer corresponds to the deeper surface of the epi- 

 dermis. Hence light passing into the eye transverses the 

 transparent deeper layers in order to reach the morphologically 

 superficial sense structures, the rods and cones,^ a condition 

 which is unlike that occurring in any invertebrate eye, with the 

 exception of the peculiar dorsal eyes described as occurring in 

 the slug OncJiidinm. 



At the place where the optic vesicle reaches the ectoderm 

 of the side of the head, the latter thicl< ' 1 \ )rtion 



of it becomes invaginated, and is 

 at last cut off as an epithelial sac, 

 — the vesicle of the leas. This 

 body, which comes to lie in the 

 aperture of the optic cup, has an 

 anterior wall of cubical cells, while 

 those of the posterior surface are 

 so strongly columnar that the cav- 

 ity is nearly obliterated. With 

 growth the cavity entirely disap- 

 pears, while the lens of the adult 

 is developed by the addition of 

 elongate fibres produced by bud- 

 ding from the cells of the equator 

 of the structure. These fibres are 

 arranged in layers, like the coats 

 of an onion, and where they meet on the inner and outer sur- 

 faces of the lens, they produce peculiar figures like a three-rayed 

 star. 



After the lens is cut off from the ectoderm the latter be- 

 comes a smooth, transparent sheet over the front of the eye, 

 forming the epithelium known as the conjunctiva, continuous 

 with the superficial layer of the skin. 



Fig. 87. Development of eye in 

 pig. t, pigmented epithelium; /, 

 lens; ni, mesenchyme; w, nervous 

 layer of retina ; r, deeper layer of 

 retina ; s, optic stalk. 



3 In man there are from 250,000 to 1,000,000 rods and cones to a square millimetre of 

 retinal surface. 



