144 Mr. P. A. Buxton on the 



specimen. I have been unable to use it with any success 

 upon Micropteryx. 



Metallic Impregnation. — The object of this method 

 is to impregnate nerve cells and fibres with actual metallic 

 silver and gold. The silver salt which is invariably used 

 is the nitrate, and as it gives excellent results I have tried 

 no other salts. The heads are dropped into a solution of 

 this salt and kept in the dark for a period. I have devoted 

 some time to discovering the best strength of silver solution 

 and the period during which the heads should be exposed 

 to its action. I find that the best results are obtained 

 by dropping them into 1 % silver nitrate in water, and leav- 

 ing them in the dark for ten days. The silver is very slow 

 in penetrating the head, and if a 6 % solution is used there 

 is great danger that the periphery will be blackened before 

 the central portions are affected at all. I believe that 

 penetration can be accelerated by keeping the whole at 

 30°-35° C. It is probable that the period during which the 

 head lies in AgNOg is immaterial provided that the fixation 

 proceeds in the dark and that sufficient time is allowed for 

 the full and equal penetration of the silver. It was not 

 found advisable to assist the silver to penetrate more quickly 

 by employing an alcoholic solution of the salt. I have, for 

 instance, experimented with a 1 % solution in 30 % alcohol, 

 following this by the various processes which I describe 

 below. The impregnation of the various fibres was not 

 obtained at all, though the various parts of the brain were 

 coloured to different degrees. In fact, the alcoholic solu- 

 tion of silver gave quite a pretty differentia] stain of no 

 particular value, but failed utterly to produce the sweeping 

 black hues which are what is desired. 



The head, then, is fixed for ten days in 1 % silver nitrate 

 in darkness. It is then washed. A few workers transfer 

 it to pyrogallic acid for a day, in order to reduce the silver 

 and leave it in the tissues in a finely divided state. I am 

 quite convinced that this is unwise. The reduction may be 

 done much more evenly by a method which I shall now de- 

 scribe. The heads are embedded, unreduced, in paraffui, 

 fixed to the slide in the usual manner, and treated with 

 xylol and descending grades of alcohol. At this stage the 

 sections are sienna-brown in colour. From a low grade of 

 alcohol the sections are moved to distilled water. (I need 

 hardly say that if the heads or sections are brought into tap 

 water a fine deposit of chloride svill be precipitated which 



