620 HOWARD. [VoL. XIX. 
IV. SUMMARY. 
1. The visual rods and cones of vertebrates represent dis- 
tinct and separable elements of a sensory epithelium. 
2. These elements are cells, usually much elongated, having 
a proximal fixed portion, containing the nucleus, and an extra- 
nuclear part ending free in the “ventricle of the primary optic 
bulb,’ or, more strictly, its morphological equivalent in the 
adult. 
3. The fixed or nuclear portion is in close contact with 
other elements of the retina lying within the membrana 
limitans externa. It possesses in addition to the nucleus, a 
basal cytoplasmic extension, the rod-, or cone-foot. 
4. The free portion (rod or cone) consists of two parts. 
distinguished by chemical and optical properties, the inner and 
outer segments. 
5. In Necturus there are present three distinctly differ- 
entiated types of visual elements called rod-cells, cone-cells, 
and double cone-cells. These have the following structure : 
RopyGErus: 
1. The outer segment has the form of a cylinder with a 
rounded distal end. 
2. A sheath is demonstrable after fixation with osmic acid. 
On the inner side of the sheath are longitudinal, parallel, 
highly refractive fibrils, twenty to thirty in number, extending 
the whole length of the segment. Usually the fibrils vary 
from the strict longitudinal course so as to form a very open 
spiral. They project slightly on the surface, so as to produce 
a longitudinal ribbing. 
3. With some stains and under certain light conditions. 
the outer segment exhibits a banded appearance, as of alter- 
nating narrow and broad transverse stripes. As cleavage 
occurs on such lines, it seems probable that the inner substance 
of the rods is arranged in plates. 
4. <A further differentiation of the inner substance is an 
