622 HOWARD. [Vor. XIX. 
of a highly refractive substance strongly chromophilic. In fixed 
preparations this is often resolved into globules. 
11. he paraboloid has the general form implied in its 
name. Distally it fits into the concave side of the ellipsoid; 
proximally it 1s often pressed against the surface of the 
nucleus, causing an involution of the latter. The paraboloid 
can be isloated and is more stable than the ellipsoid, but less 
refractive. Its substance does not stain readily. Paraboloid 
and ellipsoid must exert a definite action on the rays of light 
traversing them, because of their form and high refractive 
index, as compared with the surrounding substance. 
12. The fixed portion of the rod cell contains the nucleus 
with its very thin cytoplasmic sheath, which is continued 
proximally in the rod-foot. The nucleus is an oblong to ovate 
spheroid in form. It is larger than the other nuclei of the 
retina and contains chromatic substance, usually in a finely 
divided form. Nucleoli are usually present. In the plasmic 
sheath are longitudinal fibrils, continuations of those seen in 
the outer and inner segments. These pass into the foot, 
whence they diverge into the outer reticular layer of the 
retina. 
SINGLE CONE CELLS. 
1. ‘The outer segments have a conical form. In Necturus 
peripheral longitudinal fibrils are present, as in the rods. 
2. The interior has a banded appearance, which is due 
either to a lamellar structure or represents the edge of a 
spiral. In the goldfish, four to five spiral fibrils are present 
having a peripheral position. This system only was observed 
in the outer segment, which suggests that it may be the hom- 
ologue of the peripheral, stained fibrils in Necturus. 
3. The composition of the outer segments of the cones 
differs in several respects from that of the rods. One of 
these is the presence of a substance having a marked affinity 
for methylen blue, another, the presence of a small proportion. 
of “myeloid” substance. 
