24 BULLETIN: MUSEUM OF COMPARATIVE ZOOLOGY. 
central strand become separated (Plate 1, Figs. 40-48). If is difficult 
to say whether this is due to the action of the reagents or whether it 
is natural. In Figure 40 (Plate 4) some of the fibres are seen to lie in 
clear spaces within the sheaths while in some the sheath and central 
portion are everywhere in contact. ‘The outline of the sheaths alone 
in the former case is about as extensive as the entire structure in the 
latter case, and the central part is much smaller in the fibres where it 
is not in contact with the sheath. ‘This would lead one to think that 
shrinkage of the central bundle had occurred; but in more posterior 
sections of the same series (Plate 4, Fig. 41) there are more fibres 
which are separated from the sheaths by a space. 
On the other hand, in Figures 42 and 43 (Plate 4), which are from 
another series, and pass through the anterior part of the brain, the 
axial strands are all about equal in size, but each is separated from — 
its sheath, the extent of the separation varying in different cases. 
And in Figure 54 (Plate 5), which is from a section whose plane is 
farther back in the brain than that represented in Figure 43 (Plate 4), 
some of the fibres are distinctly separated from the sheaths, while 
in the others the sheath could not be seen. But in this case all the 
central strands are of approximately the same size. If shrinkage 
occurred at all, one would certainly expect it to affect the nerves at 
points near the eye as well as farther back, but I have never observed 
this in any series. 
It is evident from what has just preceded that the individual fibres 
of the optic nerve preserve their identity from the retinal cells to the 
brain. Figures 42, 43 (Plate 4), and Figure 54 (Plate 5) show that 
twenty-eight fibres may be distinguished some distance posterior to the 
point where the nerve enters the brain as readily as immediately 
behind the eye. Figure 54 (Plate 5) is from a section at about the 
level where the individual fibres in that series are no longer distinguish- 
able. 
The fact that there are twenty-eight cells in the eye and the same 
number of fibres in the optic nerve is strong a priori evidence that there 
is one cell for each fibre, and if the distribution of the fibres to the 
retinal cells is followed, one can hardly fail to be convinced that such 
is the case. Sections stained by Mallory’s connective-tissue method 
(Plate 5) are especially favorable for tracing the fibres. It is hardly 
necessary to state that not all preparations are equally valuable and 
that even when the staining has been particularly successful in differ- 
entiating the nerves from other tissue, it is not easy to make out the 
course of the nerves. Furthermore, if in any case the plane of the 
