STUDIES IN THE RETINA. 451 



For a clear understanding of the description and figures 

 relating to the contents of the rods to be here given, it is 

 worth while turning once more to their development, and 

 noting that, in essence, they are protoplasmic vesicles ex- 

 truded from the retina. As seen in the first part of this paper, 

 the early stages of these vesicles are seldom found intact, 

 but when they are they usually appear clear, and apparently 

 with only fluid contents. Faint traces of delicate proto- 

 plasmic networks may occasionally be seen (see Part I, PJ. 3, 

 fig. 16). Networks are, again, found iu well-preserved and 

 properly stained preparations in the large basal vacuoles of 

 the cones (see PI. 31, figs. 23, 27, 28). Later we find dis- 

 tinct networks iu the inner limbs of cones and rods, with 

 usually a certain number of very pronounced threads running 

 down in their delicate walls (see above and figs. 29, a, h, 

 i, j) ; so also in the outer limbs — which, as we saw in Part I 

 of this paper, began as fluid vesicles at the tips of the cones — 

 a protoplasmic reticulum ultimately appears. The staining 

 reticulum in the outer limbs is not often found as a simple 

 meshwork, but this is sometimes the case, and we may assume 

 that it first appears as such. Two instances are shown in 

 the figures (4, h, and 6). We gather from these cross-sections 

 that the clumps on the longitudinal threads running down 

 the rods are the points of attachment of this internal reti- 

 culum to the walls of the vesicle. As a rule this reticulum 

 is not evenly distributed ; we find a tendency for it to be 

 compressed into the axis of the rod, always, however, 

 remaining attached by its threads to the wall fibrils. As 

 this compression increases the threads of the internal 

 axial portion get very thick, coarse, and matted together. 

 The compression, may go so far that the reticulum merely 

 consists of an axial strand with a few meshes in it, while 

 the attaching threads are lengthened so as, iu cross- 

 section, to look like the spokes of a wheel (see fiigs. 12 and 

 13, h, and also cf. Hensen's optical section reproduced in 

 my fig. 7, h). 



So fai", then, the rods are protoplasmic vesicles, each 



