STUDIES IN THE RETINA. 49 



Although the rods and cones in the fish are not our special 

 subjects in this paper^ it will be necessary to enter into a few 

 details with regard to them. In the very young the elements 

 are seen to be nearly all of uniform size, with apparently the 

 same form-phases in their production as we described for the 

 Amphibia, viz. (a) small cones, (b) gradually lengthening 

 cones, (c) Schwalbe's rods, and (d) fully developed rods 

 (fig. 20, a). These are the natural stages in the formation 

 of new rods in the amphibian retina. But in the fish, 

 after the earlier stages of growth have passed, we find a very 

 striking change, which seems to begin somewhere near the 

 central region ^ and spread gradually over a large part of 

 the retina. The change is as follows : — The inner limbs of 

 the earliest formed rods gradually swell, until, in large and 

 presumably old fish, they may be of monstrous proportions. 

 So that though, while growth is still going on, there may be 

 room for a few more young cones to protrude or for a few 

 more of the inner limbs of the Schwalbe's rods to shorten 

 while the outer limbs lengthen, that time comes to an end, 

 and the retinal elements, at least over the modified area, 

 consist entirely of (a) rods with monstrous inner limbs, and 

 (b) bunches of Schwalbe's rods. The thin thread-like inner 

 limbs of the latter find their ways between the swollen inner 

 limbs of the " giant cones," while their numerous cylindrical 

 outer limbs fill up the spaces between the comparatively 

 speaking small outer limbs of the " giant cones." Fig. 20, 

 a, b, shows comparisons between the conditions of the elements 

 in young and old eyes in the viviparous blenny. Fig. 22, 

 a, h, c, shows different parts of the same retina of a young 

 plaice, a being near the centre, where it has functioned most 

 actively. Similar results might have been shown from my 

 sections of trout and stickleback. I had no sections of young- 

 cod for comparison with fig. 21, from an old fish, but we may 

 judge of the original thickness of the elements by those 

 which persist as Schwalbe's rods, a few of which are shown. 



' V/itliout having exacllj located tlie region, I believe it to be tlie pobtero- 

 ventral half of tlie central region. 



VOL. 46. PART I. — NEW SERIES. D 



