No. 3.] VISUAL: CELES, IN« VERTREBRATES:. 621 
axial portion, which is about one-quarter of the diameter of 
the rod and extends the whole length of the outer segment. 
5. The outer segments contain a substance resembling the 
myelin of the medullated nerve sheaths. Evidence for this is 
the similarity in color produced by treatment with osmic acid 
and by other tests. 
6. The outer segments are anisotropic with the positive 
optical direction parallel to their long axes. In this regard 
they agree with the axis cylinders of nerves and with muscle 
fibrils. In each of these bodies, then, the axis of maxi- 
mum elasticity 1s transverse to the longitudinal axis of the 
fiber. 
7. Outer segments in the fresh state when examined in 
eye fluids show a considerable range of contraction and elong- 
ation. If a very dilute foreign fluid is added, a rapid con- 
traction occurs, like the reaction of a living body; this is 
followed by a gradual elongation to a length greater than 
the original. Elongation of a permanent character is pro- 
duced by certain fluids of “fixing” strength. The contractility 
and transverse striation together with other characters suggest 
a possible structural analogy with muscle. 
8. Between the inner and outer segments is the inter- 
mediate plate. This structure appears as a barrier between 
the two segments, in that the continuity seems to be inter- 
rupted, except for the fibrils, which pass over the edge of the 
plate. 
g. The inner segment is in general form cylindrical, but 
less rigid in its outlines than the outer. No sheath com- 
parable to that of the outer segment seems to be present. 
The proximal portion (myoid) shows considerable con- 
tractility. Two enclosures in the inner segment take up most 
of the space. These are the ellipsoid and the paraboloid. 
Outside of these are longitudinal fibrils, which are contin- 
uations of those on the outer segment. In the inner segment 
they lie so deep as not to affect the contour of the surface. 
10. The ellipsoid varies in form from convex-concave to 
plano-concave, with the concave side proximal. It is composed 
