588 HOWARD. [Vov. XIX. 
segment. The rods (which may possibly be of two types) 
are extremely small in diameter and the staining effects so 
varied that I can not put much reliance on the appearances as 
an index of minute structure. Cross sections of the rods give 
evidence of a central core unstained by hzematoxylin as in the 
frog. 
The case of the large double cones is different. Here, in 
well preserved specimens, a constant appearance is presented 
of a system of peripheral spirals (PI. 3, Fig. 18, prs. dst.) 
made up of four or five fibrils. On the inner limb, proximal 
to the ellipsoid, straight fibrils can be traced to, and over 
the nuclei. Their number does not correspond with that of 
the spiral fibrils. The nuclei of the double cones lie close 
together and each has a distinct cone foot. 
C. FresH MATERIAL. 
In Ordinary Light. 
A thorough familiarity with elements in their normal liv- 
ing condition, or as near as possible to this state, is of the 
greatest importance in histology. This is especially true of 
investigations into the nature of the optical properties of 
living bodies, since the process of fixing or “‘killing’’ is so 
liable to affect these properties. Though gross form and 
structure may not be as sensitive to chemical alterations as 
molecular structure, yet even here, as is well known, fixation 
may do the greatest violence. As a check upon such effects, 
a knowledge of fresh material is indispensable. 
(a) RODS. 
The Outer Segments of the rods may be quite easily obtained 
by the method already described (pp. 574, 575), and a fortun- 
ate preparation may give complete rods and cones im situ. 
In Necturus, however, the cones, on account of their insta- 
bility, are not easily found in teased fresh material. 
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