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 3, 

 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-nervous 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. 



