226 BULLETIN: MUSEUM OF COMPARATIVE ‘ZOOLOGY. 
usually employed for the study of epithelial tissue. Figures 11 and 
14 (Plate 3) represent such preparations. 
In many preparations which were satisfactory for determining the 
courses of nerve fibres and the forms of the terminal structures, it has 
not been possible to make out the neurofibrillae. I have obtained 
numerous preparations, however, in which the neurofibrillae were 
easily distinguishable from the axoplasm. (See Fig. 14, Plate 8, 
and Figs. 21, 22, and 26, Plate 4.) 
The reasons underlying this selective activity, as it has been called, 
by which the silver particles become deposited chiefly in the nervous 
tissue, are as little understood as are those underlying the commoner 
methods of differential staining. It is not unusual to find it referred 
to microchemical changes, resulting from a special affinity between 
neuroplasm and silver compounds. In his discussion of the theory of 
staining, Mann (:02) states that there are some who uphold the view 
that the staining of animal and vegetable matter by dyes is a purely 
physical process, others that it is purely chemical, and still others that 
it is a physico-chemical process. While I am not prepared to state 
whether any chemical changes take place between the neuroplasm 
and the silver compounds or not, it appears to me that the contrast in 
color between nervous and non-neryvous material is too pronounced to 
permit of being explained upon a merely physical basis, such as the 
possibly greater permeability of the nervous material. Preparations 
are not infrequently obtained in which the epithelium is so little 
affected that it is scarcely visible, while the nerve courses are entirely 
blackened with a precipitate of metallic silver. It is worthy of notice, 
however, that the chemical changes which result in the blackening of 
the nerve courses are not those which take place between the nerve 
and the silver oxide, if there are such. ‘The blackening of the nerve 
courses is due to the reduction of the silver compound by formaldehyde. 
That this is true, is proved by the fact that when fresh nervous material, 
or nervous material which has been killed in absolute alcohol, is 
immersed in ammoniacal silver oxide solution there is no blackening 
of the tissue, whereas such tissue which has been killed and preserved 
in formalin is immediately changed to a dark brown when transferred 
to the Bielschowsky fluid. When transferred from this fluid to the 
20% formalin solution, the surface of the tissue becomes still more 
darkened in color. If any new compound is formed as the result of 
chemical action between neuroplasm and the silver oxide, it is a com- 
pound which behaves toward reducing agents much as silver oxide 
does. 
i 
ee 
