PLATE 4. 
(Continued from preceding page.) 
sheath, which has stained very black, is shown thicker in the drawing 
than is commonly seen. The light spaces beneath the sheath seem to cor- 
respond to the fibrils of material stained in hematoxylin. 
Fics. 35, 36.—Longitudinal and cross sections of a rod of Necturus 
fixed in osmic acid fumes. The grooving of the outer seginent appears 
as alternating light and dark lines in the thin longitudinal section (Fig. 
35). In the cross section (Fig. 36) of the outer segment the grooving 
appears as crenations of the boarder. Inside of the black sheath the 
light zone appears continuous at this magnification. 
Fic. 37——Cross sections of the outer segments of the rods from a 
frog. The fixation and magnification are similar to those of Figures 
35 and 36. No crenation of the edge is distinguishable, but an outer black 
sheath and an inner light band can be seen. 
