SILVER NITRATE. 107 



In the case of a membrane such as the epiploon, the membrane 

 must be stretched like a drum-head over a porcelain dish,* and 

 washed with distilled water in order to remove the albumi- 

 nates and white blood-corpuscles that are found on its surface ; 

 it is then washed with the solution of silver nitrate. In order 

 to obtain a powerful stain it is necessary that this part of the 

 operation be performed in direct sunlight, or at least in a 

 very brilliant light. As soon as the tissue has become white 

 and has begun to turn of a blackish grey, the membrane is 

 removed, washed in distilled water, and mounted on a slide in 

 some suitable examination medium. 



If the membrane were left in the water, the cells would 

 become detached and would not be found in the finished pre- 

 paration. 



If the membrane had not been stretched as directed, the 

 silver would be precipitated not only in the intercellular 

 spaces, but in all the small folds of the surface, and the forms 

 of the cells would be disguised. 



If the membrane had not been washed with distilled water 

 before impregnation, there would have been formed a deposit 

 of silver on every spot on which a portion of an albuminate 

 was present, and these deposits might easily be mistaken for 

 a normal structure of the tissue. It is thus that very often 

 impurities in the specimen have been described as stomata of 

 the tissue. 



If the solution be taken too weak, for instance, T500 or 

 riOOO, or if the light be not brilliant, a general instead of an 

 interstitial stain will result ; nuclei will be most stained, then 

 protoplasm and the intercellular substance will contain but 

 very little silver. 



In general, in a good " impregnation," the contents of cells, 

 and especially nuclei, are quite invisible. 



Ranvier notes that when tissues are to be impregnated by 

 immersion, they should be constantly agitated in the silver- 

 bath in order to avoid the formation on their surfaces of de- 

 posits of chlorides and albuminates of silver which would give 

 rise to deceptive appearances. 



Impregnation with silver may be followed by treatment with 

 with picro-carmine (or other carmine stain), which will bring 



* The Hoggans' histological rings, for which see below, 201, will be 

 found much more convenient. 



