No. 3.] VISUAL '‘CELES, IN: VERTEBRATES. 583 
The nuclei of the rods as well as of the other visual cells 
are larger than the other retinal nuclei and are character- 
istically elongated in a direction radial to the eye. Their 
chromation is either in the form of rather coarse or finer 
granules. The character of the nuclei no doubt changes at 
different stages of the metabolism of the cell; thus, for exam- 
ple, I have seen some evidence in experiments made by Mr. A. 
Forbes, that the distribution of chromatin varies with the 
degree of stimulation from light. I have seen nuclear division 
in embryonic material only. Such probably occurs in func- 
tional eyes of larvee at the ora serrata but never in the fundus 
of the adult retina. Nucleoli have often been observed with 
appropriate stains (Pl. 2, Fig. 7, mll.). 
Cone Cells —The outer segments of the cones in Necturus 
are extremely unstable. In some respects corrosive-acetic fix- 
ation is as favorable as any other for them; but while this 
preserves the rods uniformly, the outer segments of the cones 
give evidence of considerable distortion. The conical fort 
of these elements is quite constant, but they vary in length in 
consequence of varying degrees of contraction. The tip is 
often curled (Pl. 3, Figs. 20, 21), sometimes spirally. Exter- 
nal parallel fibrils are present as in the rod (Pl. 3, Figs. 21, 
(ane igor, (prs. dst Pl 1,Mis. 25-Pl. 5, Figsao) These 
are usually parallel to the long axis of the cone, but occasion- 
ally they have an oblique course, as is evident in Figure 23 
(Pl. 3.). Here fine oblique superficial fibrils are to be seen 
at the distal end of the outer limb. In Figure 1 the cone 
shows the same condition both on the inner segment and over 
the paraboloid. 
A central portion, which stains dark with Heidenhain’s iron- 
alum hematoxylin, is generally present in the outer limb; it 
is of varying length, but as a rule it falls short of either end. 
Though it stains heavily, the dye is easily washed out in the 
mordant. This portion in certain cases shows a plate-like 
structure (Pl. 3, Figs. 21, 23). In these instances a spiral is 
suggested, but the appearance might be explained as due te 
