STUDIES IN THE RETINA. 41 



refers to a frog tadpole, they resemble in all essentials the 

 series seen in the adult axolotl. In c four cones are crowded 

 together, the cones here, again, being far more numerous 

 than the rods. In this eye I found absolute proof of the 

 truth of my main conclusion, that the cones turn into rods, 

 for the vestiges of the refractive oil globule characteristic of 

 the cones were to be occasionally seen in various stages of 

 evanescence in the ellipsoids of the rods (see fig. 15). 



In now referring to the conditions seen in eyes under 

 5 mm., I should like to say that my examination of them has 

 not been as thorough as the difficulty of unravelling them 

 demands. The conditious vary ; in some eyes the definitive 

 elements have already begun to appear, in others one is 

 forced to the conclusion that the extrusions from the retina 

 are largely a confused mass of immense vacuoles, which 

 force back the pigmented layer. If any order exists among 

 these extruded sacs it is extremely difficult to unravel ; the 

 nearest approaches to anything like normal rods or cones are 

 long, tapering structures which take deep stains, and as to 

 which I am still quite uncertain whether they are the tips of 

 fresh extrusions forcing their way between the existing sacs, 

 or merely streaks of staining matter on the walls of the same. 

 What is clear is that the rods and cones, such as they are 

 seen in the adult, or even in eyes over 1 mm. in diameter, may 

 not be visible ; and yet there can be little doubt that out of 

 these large sacs the future retinal elements will be developed. 

 It is interesting further to note that the pigment granules 

 which climb up between these sacs do not seem to be able to 

 retreat back again to the pigment layer, as is the case in 

 eyes with definitive rods and cones if kept in the dark before 

 fixation. The clinging of pigment to the young cones, i. e. to 

 the raAV material of the rods, has been already noted (see p. 27). 

 From this stage, in which the retinal elements are largely 

 undifferentiated sacs, filled apparently simply with fluid, the 

 eye passes only gradually. The process has still to be worked 

 out in detail. We will here pass on to note that in much 

 older eyes, after normal rods and cones have appeared, we 



