166 Mr. H. M. Bernard on 



these degenerate eyes appear usually to escape into the cavity 

 of the eye, in some cases at least, unaltered *. 



Turning to the vertebrate eye proper, we are justified 

 in affirming that, however highly specialized, it is a product 

 of the same activities as have given rise to the simpler 

 eyes above described. With regard to its structure, what 

 chiefly concerns us is the fact that the sensory cells are 

 turned towards the advancing streams of granules. I feel 

 justified in speaking of the streams of granules, because, as 

 is known, not only are these granules carried about in cells, 

 but they are apparently capable, under tiie action of stimulus, 

 of independent movements within cells, and, further, can pass 

 from cell to cell. Amoeboid movements have been claimed 

 for the individual granules from the eye of vertebrates when 

 floating free in a suitable medium. That the granules in 

 many eyes are highly complex bodies (perhaps secondarily 

 specialized), and not simple concretions, follows from the fact 

 that they seem to contain a staining kernel of chromatin, and 

 in the eye of the crustacean Apus they are invested iu a fine 

 hyaline layer of matter and can be found dividing f. 



Passing by cornea, iris, lens, and other accessory structures, 

 we may say that the vertebrate eye, as a sense-organ, consists 

 essentially of a thick layer of nerve and sensory tissue effec- 

 tually opposing, or only greatly impeding, the advance of 

 granules contained in the cells of an epithelium which is in 

 contact with this sensory layer over its whole extent. These 

 granules should normally reach the surface of the body and 

 take part in the formation of the external protective structures. 

 They are, however, kept back, but are stimulated to renewed 

 efforts every time the light falls on them. This attack on 

 the nervous-tissue layer, to try and force a way through 

 whenever attracted by the light, I believe to be the secret of 

 our light sensations. 



We may point out here that albinism, which has hitherto 

 prevented physiologists from seeing in the movements of the 

 pigmented granules any essential factor iu the production of 

 light sensation, is no difficulty to this theory J. The granules 

 themselves are doubtless there in some form or other, only 

 they lack the colour. This deficiency may be quantitative or 



* Cf. W. Baldwin Spencer, " On the Presence and Structure of the 

 Pineal Eye of the Lacertiliaus " (Q,. J. M. S. xxvii. 1887). 



t In conjunction with the norma) granules others of such purely excre- 

 tory matter as guaniu are found iu some eyes. This phenouieuou is quite 

 in accordance with our theory. 



X The seductive analogy between the eye and a photographic camera 

 with its sensitive films has also had something to do with drawing a ten- 

 tion away from the pigmented granules. 



