STUDIES IN THE RETINA. 457 



noticed also that the outer limbs of Schwalbe's rods {i\ and?-,) 

 were in most cases rather more deeply stained than the longer, 

 thicker definitive rods, although I lay no great stress on this. 

 The accidents which can never be eliminated from our technical 

 methods are too numerous to allow conclusions to be based 

 upon mere variations in diffuse staining. I mention the point, 

 however, jnst because it is possible that the proportion of the 

 refractive matter to the staining reticulum might be expected 

 to be less in an outer limb, just beginning to fill up, than in a 

 large swollen rod. It is this refractive matter which gives the 

 rods their characteristic appearances, and which has led to 

 their being classed among cuticular structures. 



The source of this refractive matter is to be seen in the 

 pigment epithelium into which the tips of the rods are pluuged, 

 and it is largely composed of pigment granules, probably with 

 some portion of the protoplasm of the epithelial cells. At 

 least the absorption of cytoplasm as well as pigment by the 

 rods can actually be shown to take place under special cir- 

 cumstances, as we shall presently see. 



In the first place, dealing for tke moment with general 

 considerations, I again refer to the development of the rod ; 

 a fluid vesicle is thrust into the pigment layei', and slowly 

 becomes filled with refractive matter. Both the vesicle and 

 the epithelial cells are, so far as we can see, naked proto- 

 plasm in the very closest contact with one another, — indeed, 

 tightly interlocked, the pigment cells constantly forcing a 

 passage up between the packed rods.^ Between these some 

 interaction is almost certain to take place. This interaction 

 is, I contend, in part at least an absorption of pigment by 

 the rods. The pigment of the epithelial cells is constantly 

 recruited by an outward streaming of granules from the 

 choroidal layer adjacent to it, a streaming which can be seen 

 in every successful preparation. So that we may conclude 

 that pigment is being used up and as constantly replaced. 

 The only other alternatives to this view are either that the 

 refractive matter in the outer limbs of the rods comes from 



' For the evidence that the rod layer is normally compact see Part ]. 

 VOL. 44, PART 3. — NEW SERIES, G G 



