6 ATLAS OF NERVE CELLS 



employed without producing any difference in the results. In other lines of work I have 

 even used a 4 % and a 10 % solution, the effect apparently being to increase, sometimes, the 

 number of elements impregnated with a tendency to lose, to some extent, cleanness of 

 impregnation. 



The pieces are brought immediately into the silver solution, and the first solution used may 

 be one that has been used in a previous impregnation. The pieces are simply rinsed in this 

 first solution, which need merely be sufficient to cover them. A copious precipitate is imme- 

 diately formed, and then the fluid is poured off and thrown away. When a second quantity 

 is poured upon the tissue, it usually clouds up slightly. This is again poured off, and the 

 pieces are now brought into the final solution. This may again become discoloured, especially 

 if the pieces be of some size, and should then be changed. The quantity of the solution in 

 which the pieces finally rest should be considerable, even double the quantity of osmic-bichromate 

 used in hardening. When brought into the final fluid, the pieces are placed upon absorbent 

 cotton to facilitate the diffusion of the silver nitrate into the tissue. 



The impregnation begins very soon. Golgi says it may begin immediately at the surface 

 and be completed in favourable cases in 2-3 hours. It is generally completed in 12-24 hours. 

 When the pieces cannot be cut at once they should be kept in the silver solution, and this 

 should be changed whenever it shows a yellowish or greenish tinge. Tissues can be thus 

 kept in silver for several weeks or even longer, though it is wise to cut them sooner. 



Usually both hardening and impregnation are done in the dark. This is not necessary 

 however. The light plays no part in the impregnation. When the pieces are to be kept in 

 the silver for some time, it is better to keep them in the dark. At any stage in the pro- 

 cedure they may be brought into the light temporarily for examination, etc., without any fear 

 of injury. 



Besides the precipitation of silver chromate, and possibly of other silver compounds in 

 the tissue, another change effected by the silver, to which I have not observed any allusion, 

 is the clearing up of the blackening produced by the osmic acid in the hardening. 



As above noted, one difficulty with the Golgi silver methods, is the slow and feeble 

 penetration of the osmic-bichromate (in the rapid method), and also of the silver nitrate, 

 so that impregnations are limited to a certain distance from the surfaces of the pieces 

 besides being often more unequal than is desirable. The writer sought to overcome the latter 

 and increase the diffusion of the silver by adding sulphates to the silver solution. The sulphates 

 of sodium and zinc when thus added did not interfere with the impregnation, but whether 

 they produced the desired effect to any extent is questionable. Their presence in some of the 

 silver solutions given below need not be regarded as one of the requisites to obtaining the 



