610 HOWARD. [Vor. XIX. 
piates to wave lengths of light by some diffraction phenomenon, 
then expansion or contraction of the outer segment would 
necessarily affect the result by varying distances between plates. 
Since I have not as yet systematically investigated the 
physiology of visual cells, my suggestions are purely tentative. 
Interesting as such considerations are, it must be granted that 
they can be little else than speculations until we have obtained 
a more complete knowledge of the structure of visual cells 
as well as further information as to their chemical and physical 
characters. 
I wish now to discuss what I have called a “central core.” 
The differentiation of this central core or axis in the outer 
segment occurs so frequently that I think we cannot but assume 
its existence in the living condition. When differentiated at 
all, this feature occurs in fixed and stained preparations as 
an unstained portion. On this account Greef (: 00) explained 
the appearance as a fixation effect and due to difference of con- 
sistency between the contents of the outer segment and its 
sheath. This hypothesis, however, does not explain the selec- 
tive staining of the core by toluidin blue in fresh material, a 
fact which I consider of great weight. The irregular central 
staining mass in rods treated by Mallory’s (: 00) triple stain 
is probably not identifiable with this body, since its distribu- 
tion is inconstant and apparently limited to the center as a 
result of shrinkage only (Pl. 5, Fig. 38). 
The more regular and constant appearance which I have 
mentioned as an axial core suggests the axial fiber, about 
which there has been so much controversy. This has beer 
variously described, and, because of ambiguity in terms, it is 
scarcely possible to be certain, except from figures, to what 
the observations really referred. Probably the best known of 
these was the axial fiber described by Ritter, the presence of 
which was confirmed by Manz, Schiess, and Hensen, but not 
generally admitted by others. Schultze gave the question 
considerable attention, reporting (’724; ’72>, p. 821) observa- 
tions of his own on fresh mammalian material. He finally 
