144 S. WATASE. 



longer any doubt that a simple monostichous structure characterises the 

 lateral eyes of Scorpions as originally shown by Bourne and myself. It is 

 perhaps well to note that Grenacher was the first to describe the charac- 

 teristic monostichous structure of the lateral eye of Limulus, on which Mr. 

 Watase's theory is based. 



Interesting matters for investigation and speculation are opened up by Mr. 

 Watase's views. Such matters are the relation of the diplostichous mono- 

 meuiscous central eyes of the Arachnida to compound or polymeniscous om- 

 matidial eyes of Arthropoda generally. And especially interesting, it seems 

 to me, would be the attempt to account for that incipient segregation of the 

 retinal cells into groups united by a five-fluted rhabdom which we find in 

 both the lateral and the central eyes of Scorpio. 



March, 1890. E. Eay Lankester. 



Balfour^ has given a sketch of the possible evolution of a 

 visual organ. He starts with a simple organism in which a 

 spot on the surface of the body may become spontaneously 

 pigmented and therefore become specially sensitive to light. 

 The cuticular covering of the body may become thickened at 

 this spot and act as an apparatus for condensing the light upon 

 the pigmented spot lying beneath it. He further expresses 

 his view elsewhere/ that the lens-like dioptric apparatus of 

 the eye, formed either as a thickening of the cuticle or as a 

 mass of cells, was at first formed simply to concentrate the 

 light on the sensitive spot ; the power to throw an image of 

 external objects on the perceptive part of the eye was acquired 

 gradually afterward. 



The part which is played by pigment in the physiology 

 of vision is considered a most obscure problem. I quote 

 the following, clearly put forward by Foster,^ as a physiological 

 aspect of the question bearing upon the discussion at issue : 



1 F. M. Balfour, " Address to the Department of Anatomy and Physiology, 

 British Association, 1880," 'Nature,' vol. xxii, p. 417, 1880. 



' ' Comparative Embryology,' vol. ii, chap, xvi, " Organs of Vision," p. 

 470. 



^ M. Foster, ' A Text Book of Physiology,' 4th edition, book iii, chap, ii, 

 " Sight: the Photochemistry of Retina," pp. 515, 516. 



