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X. — Note on Cajal's Formalin- Silver Nitrate Impregnation 

 Method for the Golgi Appai'atus. 



By H. M. Carleton, B.A., Demonstrator in Histology, and 



Christopher Welch Scholar, University of Oxford. 



(From the Department of Physiology.) 



{Read November 19, 1919.) 

 With Three Text-Figures. 



1. Silver Impregnation in General. 



By impregnation is usually meant the deposition of metallic salts 

 in tissues. In its simplest form small pieces of tissue are treated 

 with the salt in watery solution (e.g. silver nitrate), after which 

 the metallic component of the salt is reduced as a coloured 

 compound by the action of light. And owing to differences in the 

 degree of affinity of various tissue elements for the metallic salt, 

 such an impregnation is often differential. Thus the above 

 method, originally introduced by Von Kecklinghausen {10)* in 1860, 

 impregnates the inter- cellular substance of endothelial cells, etc., 

 black. Such crude methods, however, soon gave way (except for 

 special purposes) to others more precise. Thus it was found that 

 fixation before the silver bath increased the selectivity of the metal 

 for certain tissue constituents by acting in respect of these as a 

 mordant. The methods of Golgi {6) for the impregnation of axis- 

 cylinders and dendrites of the central nervous system are examples 

 of this. Finally, the so-called photographic methods came into vogue 

 for certain purposes. In these the tissues undergo : — 1. Fixation. 

 2. Impregnation with the silver-salt in solution. 3. Reduction of 

 the same, usually by means of a modified photographic developer. 

 4. Toning by immersion in a solution of gold chloride, thereby 

 inducing a partial substitution of the latter metal for the silver. 

 ' Frequently sodium hyposulphite is added to the toning bath with 

 a view to dissolving any unreduced silver. 



The Cajal methods for neuiofibrils (J), and the Golgi (7) and 

 Cajal impregnations for the Golgi apparatus, embody the above 

 processes. 



The differences between impregnation and staining are often 



* The italic figures within brackets refer to the Bibliography at end of the 

 paper. 



Z 



