260 VASIL OBRESHKOVE 



ments on eye and skin and on skin alone. The rest of the appara- 

 tus was the same as shown in figure 3. With the aid of a dim 

 Hght, the end of the cone was closely applied in the direction of 

 the eye lying next to the side of the aquarium, so that when the 

 Hght was turned on, the rays reached the eye only. Such an 

 arrangement made it possible to illuminate the eye by an accu- 

 rately measured intensity of light. 



The sensitivity of the eye was studied in detail. Several 

 individuals, which had previously responded with a great degree 

 of regularity, were used. Repeated stimulations of the eye at 

 one-minute intervals produced no response. If, however, the 

 cone was slightly rotated or its position moved forward or back 

 in the process of adjustment, occasional responses were observed. 

 Such reactions were attributed to the stimulus of motion rather 

 than to the influence of light, for the same degree of stimulus 

 accompanied with no motion led to no response. 



In the eye of the tadpoles, hbwever, we find an apparatus 

 adapted for receiving light stimulus. It is not to be questioned 

 that during illumination chemical changes are occurring in the 

 retina. In fact, if we consider the electrical changes in the 

 retina due to light of a photochemical order, as demonstrated by 

 Dewar and McKendrick (74), Waller ('00), Einthoven and 

 Jolly ('08), we have a chemical change in the retina in a given 

 time dependent upon the intensity of illumination and measured 

 by the amount of electrical current produced. This has been 

 demonstrated by all the workers on retinal changes. The lack 

 of responsiveness of Rana clamitans tadpoles when the stimulus 

 is appHed only to the eye and the orderly responses when the 

 skin is stimulated becomes clear when we consider the difference 

 in specialization and organization of these two types of receptors. 

 The skin receptors are connected with the lower reflex centers, 

 while the eye is a more specialized analyzer for image-forming 

 functions. The distinction in function is made clear by Pavlov, 

 who pointed out that in the lower centers the reflexes are un- 

 conditional; that is, knowing the nature of the stimulus, the 

 responses can be predicted. When the stimulus is apphed to 

 a higher, more specialized sense organ, the occurrence or a 



