130 VERTEBRATE PHOTORECEPTORS 



maximum about 575 m/z in contrast with that of rhodopsin 

 whose maximum is about 500 m/x. 



It can be said that up to the present time only three ret- 

 inal photopigments have been identified, viz., rhodopsin and 

 porphyropsin, which are rod photopigments, and iodopsin, 

 which is a cone photopigment. All vertebrate visual systems 

 have been built upon the carotenoid pigments. The synthesis 

 of these pigments, as far as is known, depends upon the pres- 

 ence in the diet of a carotenoid in the form of a vitamin. Vita- 

 min A bears this relation to rod vision in most animals and 

 where deficiencies arise, night blindness results. 



Whenever one finds marked physiological or chemical 

 changes taking place in a system one naturally seeks for 

 some histological evidence of these changes. In this connec- 

 tion might be cited, among other findings, the so-called 

 Kolmer's 'droplets.' In 1909 Kolmer found that when the 

 retina was preserved with a dichromate fixative containing 

 acetic acid and stained subsequently with iron hematoxylin, 

 there occurred on and between the visual cells deeply staining 

 granules or 'droplets' (Figures 84-88). In dark-adapted 

 retinae these granules were abundant, whereas in light- 

 adapted retinae they were sparse. Since the administration 

 of pilocarpine brought about an increase, even in light- 

 adapted eyes, he stressed their secretory nature and their 

 origin from the pigment epithelium. 



Although Kolmer originally was inclined to regard the 

 droplets as artifacts, he finally concluded that they were 

 secretory products of the pigment epithelial layer. His 

 observations led him to point out several correlations between 

 the droplets and visual purple, viz., 1) the presence of drop- 

 lets on the outer segments or on the processes of the epithelial 

 pigment cells, the latter regarded at that time as the source 

 of visual purple; 2) their absence in pure cone retinae of 

 diurnal lizards in which visual purple is absent; 3) their 

 abundance in dark-adapted retinae containing rods, wherein 

 visual purple is also at its maximum; 4) their sparsity in 

 light-adapted retinae, in which visual purple is absent or 



