STUDIES IN THE EETINA. 45 



fire small and round their outlines are cleai-, being preserved, 

 no doubt, by the fixing of the pigment cells on the one hand, 

 and of the deep staining matter, which usually forms their 

 proximal walls, on the other (see figs. 17 and 18, where the 

 shaded vesicles represent deeply stained walls) . As soon, how- 

 ever, as they lengthen out, the walls become so delicate that 

 they collapse under the violent processes of fixing, hardening, 

 and preparing the sections. It is common to find in young 

 eyes great empty spaces where the rod layer should be 

 between the retina and the pigment, the spaces occasion- 

 ally interrupted by single, short, thick, deeply staining 

 rod-like structures, one here and there having survived. 

 That elements of some form or other filled these gaps is 

 absolutely certain ; indeed, the ragged remains of membranes 

 can often be seen fringing the distal ends of the nuclei, and 

 protruding a little from the membraua limitans externa. A 

 great many sections show nothing but this, and one is apt to 

 become hopeless of ever seeing the vesicles which, in life, had 

 been crowded together in those gaps. On one occasion I 

 found one of these spaces occupied by a single large 

 vesicle with a complete pigment cell, which had left the pig- 

 ment epithelium, inside it. In time traces of long vesicles 

 become more frequent because they are supported and 

 preserved by being in contact with other more formed and 

 stronger elements (Part I, PI. 3, fig. 16). It is when a 

 number of very fragile vesicles are mutually supporting and 

 squeezing one another that they disappear from our sections 

 leaving hardly a trace behind. 



In sections of retinas killed at night I have succeeded at 

 last in finding vesicles intact. I'hey are slightly mottled 

 and dotted over with stain, and I conclude that they owe 

 their preservation largely to this fact, viz;, that their walls 

 were strengthened by this staining matter, as appears to have 

 been the case with the rods and the cone tips in the retinas 

 above referred to from Table Mountain. Fig. 12 shows a 

 group which have fortunately been preserved intact, and 

 fig. 13, a — e, are elements from the same retina. 



