PADRE Ss 
Figures 18, 19, 21, 23 and 24 are magnified 1,450 diameters; Figure 
20 is magnified 1,000 diameters. 
All the preparations, except that shown in Figure 22, were fixed in 
corrosive-acetic mixture and stained with Heidenhain’s iron-alum-hzema- 
toxylin; all are from Necturus, except Figures 18, 109. 
Fics. 18, 19.—Visual elements of goldfish seen in radial sections of 
the retina. In Figure 19 the three different types of elements, rods, cones 
and double cones are shown as they occurred in actual section. ‘The rods 
with long, thread-like inner limbs occur at varying distances from the 
membrana limitans externa. The double cones with long inner limbs are 
more distal than the single cones. The outer nuclear layer consists of four 
or five strata of large cone and smaller rod nuclei. 
In Figure 18 a double cone is shown in detail, only one of the 
couplet, however, being in focus. In the outer limb two to four spiral 
bands are seen. These do not correspond in number or in size with the 
straight parallel fibrils of the inner limb, seen just distal to the nucleus. 
The light area proximal to the ellipsoid is probably the homologue of the 
paraboloid in amphibians. 
Fic. 20.—A portion of retina embracing the pigment layer and part 
of the outer nuclear layer. A rod and single cone are shown in detail and 
a double cone in outline. The end of the outer segment of the rod is sur- 
rounded by processes from the pigment cells. The nuclei and inner seg- 
ments are in optical section; the outer segments in superficial focus. 
Fic. 21.—The near cone of a double cone showing plate-like structure. 
Vacuolation on opposite sides between alternate “plates” gives a spiral 
appearance. Superficial fibrils are visible at the Proximal end of the outer 
segment. 
Fic. 22.—A fresh cone in fluids from the eye and shortly after removal 
from the eye. The animal from which this was taken had been kept in 
the dark three days. This treatment induced elongation of the cone, but 
probably not to the extent seen here. The extreme elongation is a char- 
acteristic form assumed by the outer segments of cones upon the death of 
the organism. This outer segment gave the negative reaction with polar- 
ized light, being yellow when at right angles to the a axis of the gypsum 
plate. 
Fic. 23.—A cone typical of heavily stained material. In the center 
of the outer segment are structures which do not persistently retain the 
hematoxylin stain. The appearance is that of plates connected by a lightly 
stained axis. Their oblique relation to the latter suggests a spiral, made up 
of a continuous band or broad fiber. However, an entirely satisfactory 
demonstration of such a spiral has not been secured in Necturus. At the 
distal end of the outer segment superficial fibrils are visible, which take a 
direction slightly oblique to the long axis of the cone. Between the 
nucleus and ellipsoid the superficial fibrils are straight. ‘The paraboloid 
is drawn a little more distinctly than it would appear if the fibrils had been 
in focus. 
Fic. 24—Outer segment of a rod showing evidence of “plate” 
structure of great regularity. 
Fic. 25.—A portion of a cone. Distinctly staining, superficial fibrils 
are visible on the fragment of the outer segment. The ellipsoid is densely 
stained with hematoxylin. 
