46 VERTEBRATE PHOTORECEPTORS 



Designating a visual cell as a rod or a cone on morphologi- 

 cal grounds alone does not seem entirely useless. Anyone 

 who has made an extensive histological study of the verte- 

 brate retina soon becomes convinced of the existence of two 

 different cellular types. However, there are other criteria 

 which one can use if one is not entirely convinced by morpho- 

 logical evidence alone. Those who have had long experience 

 with different staining methods are able to distinguish these 

 two types of visual cells by their different chemical affinities. 



Most of the work on differential staining of the visual cells 

 has been done by Kolmer (1936). With Unna's orcein- 

 polychrome methylene blue-tannin stain, he colored the 

 outer segments of the cones deep blue, whereas the outer 

 limbs of the rods remained unstained — indicating a physico- 

 chemical difference. With Mallory's stain, a distinct differ- 

 ence was seen in the cone and rod nuclei; the cone nuclei 

 stained red with fuchsin, the rod nuclei stained orange. 

 After fixation in chrome-containing fluids and treatment 

 with nascent chlorine, Unna's stain colored the inner and 

 outer portions of the cones a deep blue (with Wasserblau) ; 

 the corresponding portions of the rods were colored red (with 

 safranin). These and still other methods demonstrated, in 

 many vertebrates, a distinct difference between rods and 

 cones. Wolff (1940), from whom the above information was 

 taken, also states that he himself succeeded, in the human, in 

 coloring the inner portions of the cones red with Mallory's 

 triple stain after Zenker fixation, whereas the corresponding 

 portion of the rod stained blue. This is of especial interest, 

 for he found that the inner limbs of the rod-like cones stained 

 as cones (red) and not as rods. 



It is apparent, as more work is being done on the photo- 

 chemistry of the retina, that the photopigments in the two 

 types of cells are different. This also constitutes a strong 

 argument in favor of the separateness of the visual elements. 



Lastly, one must not fail to offer as additional evidence for 

 the discreteness of visual cells the inverse photomechanical 

 reactions which the cone and rod myoids exhibit in many 



