STUDIES IN THE RETINA. 33 



inner limb to its normal length of 10 ju. Here I should again 

 like to emphasise the fact that although I use these exact 

 figures, they are only approximate ; lengths of outer limbs 

 between these occur, but the numbers given occur with 

 sufiicient frequency to convince one that there is some under- 

 lying law which has to be discovered. After adopting and 

 abandoning several possible explanations, I am inclined to 

 regard this curious fitting of the successive stages in the 

 development of the rods into the scale made by sections of 

 the cones as due to the varying degrees of lateral pressure 

 exerted by these latter. This pressure will be greatest in 

 the zone of the innermost cone section and less in that of the 

 next, and so on. This explanation was suggested to me by 

 noting the similarity between the inner limbs of Schwalbe's 

 rods and the conditions invariably presented by one of the 

 two cones in every pair of ^' double cones." I give several 

 drawings (fig. 5, a — ■/) of these so-called double cones as seen 

 in the retina of the frog, about which so much has been 

 written. I Of these figures only a shows the full length of the 

 cones; the rest, being from sections prepared by other fix- 

 tures than heat, have lost their tips. I can myself find little 

 more in them than that two cones develop side by side, and 

 one of them is generally somewhat distorted by the pressure 

 of the other, the pressure being exerted in many cases by the 

 large basal vacuole. The peculiar appearance of the '^ twin 

 cones" (at least in the frog) is thus of the nature of an acci- 

 dent, and a very simple accident. Whichever cone first 

 develops its basal vacuole squeezes all the material out of 

 the basal section of its companion into its distal end, as is 

 shown in the figures. If the inner limbs of the left-hand 

 cones in a and/, or of the right-hand cones of h and c, are 

 compared with the inner limbs of Schwalbe's rods (fig. 2, c 

 and Z), we see that pressure could easily account for the shape 

 of the latter.^ It seems, then, that if the cone at stage c^ in 



^ These long-limbed cones which always form one of the " double cones " 

 are the only elements I can find at all corresponding with the long-limbed 

 VOL. 43, PART 1. NEW SERIES. C 



