608 HOWARD. [Vor. XIX. 
several ways. Its inconstancy of appearance in stained mate- 
rial is probably due to a uniformity in the laminz or bands 
of an individual rod, which prevents differentiation under 
usual conditions. I have noted its appearance most frequently 
with methylen blue in fresh rods. Rarely transverse striation 
is to be seen in material stained by hematoxylin. Such a 
preparation is shown in Fig. 24 (PI. 3). The cementing sub- 
stance (Kittsubstanz, Greef, :00) is here evidently stained 
by the hematoxylin. This condition, in connection with other 
observed phenomena, has led me to suspect (see pp. 581, 596- 
597) that these excessively thin layers may be nerve conduct- 
ing fibrils lying between thicker plates (catoptric in function 
or for isolation), and connecting on the outside of the rod 
with the longitudinal fibrils. Such a structure would seem to 
agree in general with that made out in invertebrate eyes 
(Schultze, Hensen, Patten, Parker, Hesse). I think we must 
agree that Schultze was right when he said: “However dif- 
ferent the formation and development of the eyes of animals 
may be in general, still for the purpose of transferring the 
undulations of light to the domain of nervous conduction we 
may assume a conformity in the structure of the terminal 
organs” (Schultze, ’72*, p. 1006, ’72, p. 826). The pres- 
ence of transverse nerve fibrils has not been demonstrated, 
but such a condition would seem to be consistent with some 
polarized light reactions, and a few other observations, as I 
have already recorded (pp. 581, 596-597). 
This hypothesis would, however, have to be supplemented 
to account for such further phenomena as the high degree 
of contractility and the alternating layers of optically different 
substances. These suggest an analogy in structure to muscle 
fibrils. 
Direct observation of contraction in the outer segments of 
rods, such as I have reported, has not, I believe, been pre- 
viously recorded. Several workers, however, have given the 
results of measurements on the visual elements examined in 
animals subjected to different degrees of light and darkness, 
