142 VERTEBRATE PHOTORECEPTORS 



there was extensive migration of epithelial pigment (Figure 

 100, cf. Figm-e 95). 



The results of our experiments on the whole have shown 

 that droplets are usually abundantly present in frog retinae, 

 light-adapted at 0°C (following dark-adaptation at room 

 temperature), and that they are present in small numbers 

 in retinae dark-adapted at 0°C (following light-adaptation 

 at room temperature). The results thus strengthen the 

 theory that the droplets may represent a histological picture 

 of retinene. The droplets and retinene have been shown to 

 exhibit common properties and similarities of behavior. 

 These may be cited as follows: 1) site of occurrence, 

 2) lipoid nature, 3) absence in extreme avitaminosis, 4) pres- 

 ence in the retinae of animals dark-adapted at room tempera- 

 ture and treated with certain acids and heavy metal chlo- 

 rides, 5) absence or sparsity in retinae light-adapted at room 

 temperature and treated as described above, and 6) a 

 parallel behavior at 0°C under various conditions of illumina- 

 tion. It might be pertinent to point out here that in the 

 light-adapted intact eyes at room temperature, retinene is 

 converted to vitamin A, but that two-thirds to five-sixths 

 of the vitamin A has diffused out of the retina (Wald, 19356, 

 1936). It would appear likely, therefore, that vitamin A 

 might form droplets as readily as retinene; but it is this 

 diffusion out of the retina that has actually removed most 

 of the formed vitamin A. Thus the few droplets seen in 

 light-adapted retinas may represent this residual vitamin A, 

 in addition to retinene, rather than retinene alone. 



The results of our investigations suggest that the droplets 

 are histological entities which may be identified with reti- 

 nene ; since both droplets and retinene show a striking simi- 

 larity in behavior under similar conditions of illumination 

 and temperature, and possess a number of other properties 

 in common (v. Table 3). 



