174 PRACTICAL HISTOLOGY. 



lymphatics and bloodvessels are most numerous, the 

 epithelioid layer is deficient, or at least incomplete 

 and modified. 



The salt solution is now thoroughly washed from 

 the membrane with distilled water, and without 

 delay a quantity of nitrate of silver solution is 

 poured on it and allowed to run over every part of 

 the exposed surface. After five minutes it is again 

 washed with distilled water, and the glass plates, 

 with the membrane of course still upon the surface 

 of the upper one, are placed in the weak spirit con- 

 tained in the flat glass dish. The dish is then 

 covered and placed in the sunlight until the silver, 

 as evidenced by the change in color, is fully reduced. 

 Small pieces, each about an inch square, are then cut . 

 with sharp scissors from various parts, floated on to 

 slides with the browned surface uppermost, and are 

 exposed to the air for a few minutes to allow most 

 of the spirit to evaporate, leaving them in water ; 

 to this a drop of glycerine is added, and finally the 

 cover-glass is superposed. Or the cover-glass may be 

 put on first, and then glycerine placed at the edge 

 and allowed to diffuse in underneath the cover. The 

 preparation is completed by cementing the covering- 

 glass in the usual way. 



Both before and after the treatment with nitrate 

 of silver it may have been noticed that the delicate 

 membrane is studded all over with patches of thicker 

 tissue, some quite small and insular, others extend- 

 ing over a considerable area. These patches, which 

 are characterized by an accumulation of lymphoid 

 cells and by the small size of the epithelioid cells of 

 the surface (there being, in fact, in many cases, a 

 loss of distinction between them and the other cellu- 

 lar elements of the membrane), are, at least the 

 larger ones, provided with numerous bloodvessels, 

 the epithelioid lining of which is often very well 

 stained by the silver; and these are always accom- 

 panied by one or more lymphatic vessels, with walls 

 formed by the characteristic wavy outlined cells. 



