Observations on Retina of Gecko 



215 



certain observations have been made which suggest that the outer 

 segment of the rod may be more directly concerned in the pro- 

 duction of this pigment. 



When the retina is preserved in modified Held's fixative (v. 

 Kolmer, op. cit.) and stained with iron haematoxylin and 

 erythrosin, numerous droplets, which stain with a varying inten- 

 sity from gray to black, are seen on and between the outer seg- 

 ments of the rods. In addition, a characteristic lamellar arrange- 

 ment of deeply staining granular material in the outer segment 

 of the rod is very evident. When stained sections of eyes which 

 have been kept in darkness for twenty-four hours are studied 

 microscopically, the droplets are found to be very numerous and 

 the deeply stained granular material in the rod outer segment is 

 greatly reduced in amount or is not to be seen at all. On the 

 other hand, identically prepared sections of eyes that have been 

 exposed to light for six hours not only show a great reduction in 

 the number of droplets, but the granular striations in the rods are 

 very compact and deeply stained. Intergrades between the two 

 extreme conditions are also to be found and, within the same re- 

 tina, whether light or dark adapted, there is a stiking correla- 

 tion between the number of droplets present and the amount of 

 granular material within the rod outer segment. 



Kolmer described the presence of these droplets in the 

 retinae of various vertebrates. In the frog retina kept in 

 darkness, he found these droplets to be larger and more numer- 

 ous than in the illuminated retina, and after illumination with 

 direct sunlight, they were not to be seen at all. These droplets 

 are ascribed by Kolmer to be secretion products of the retinal 

 epithelial cells and he makes no mention of a relationship exist- 

 ing between the number of droplets and the amount of granular 

 material in the rod outer segment. His failure to find them in 

 the pure cone retince of lizards and snakes, strengthened his as- 

 sumption that they are concerned in rod vision and perhaps with 

 the appearance of visual purple. 



From the study of these droplets in the gecko retina, two sig- 

 nificant points are brought out. The number of droplets outside 

 the rod seems to be inversely proportional to the amount of 

 granular striated material within the rod outer segment. Their 

 genesis from the rod, therefore, seems to be less questionable 

 than an origin from the pigment epithelial cells. Further, since 



