389 



the silver injection, an error readily corrected by combining 

 gelatine Avitli the solution of silver injected, the vessels are 

 thus equaly and smoothly distended, yet the epithelium 

 appears mapped out as usual. 



Feltz asserts that if a solution of silver be allowed to dry 

 in the light on a coUodium film, irregular black lines are 

 produced, quite like those observed after its action on organic 

 membranes. I myself have examined the irregular figures 

 produced by this experiment, and cannot conceive how any 

 one accustomed to the precise study of organic forms can see 

 any similarity between them and the definite outlines pro- 

 duced by the action of silver solutions on epithelial surfaces. 



Besides the preparations exhibiting the vascular epithelium 

 ■which have been described, the Museum possesses, as I have 

 already mentioned, a number in which the epithelium of the 

 skin, of the ly^mph-sacs of frogs, of the peritonseum and of 

 other surfaces, are mapped out by silver staining, and the 

 reagent is continually employed by myself and my assistants 

 in the investigation of such surfaces. It is impossible for any 

 one who has had such opportunites for observation, to avoid 

 being struck by the fact that the outlines obtained have a 

 definite form and character for each tissue. It is true that 

 the silver staining does not succeed as frequently as carmine 

 staining does, that its use requires more skill and that failures 

 are more frequent. Sometimes too much action takes place 

 and everything is obscured by the black precipitate produced ; 

 sometimes either because the tissues are not fresh, or the 

 light not sufficient, or from some unexplained reason, the 

 solution does not act at all ; but the forms above described as 

 characteristic of the arteries are never observed in the veins, 

 never do the outlines produced on the surface of the skin 

 resemble those seen on the pei-itonanun, in the lymph- sacs, 

 or in the vessels ; each membi'ane permits only the formation 

 of its own characteristic outlines, never of those belonging to 

 another tissue ; moreover, in all cases where it is possible to 

 observe the shape of the epithelial cells without the use of 

 reagents, or to isolate them, the forms thus ascertained cor- 

 respond precisely with those mapped out by the siver solu- 

 tion, and when, after the action of silver, carmine staining is 

 resorted to, the nuclei thus made visible correspond in posi- 

 tion to the places they ought to occupy, if in fact the silver 

 had mapped out the cell-boundaries as I certainly believe it 

 does. Whether the discoloration is in the cell wall, or in 

 the cement or matrix by which the adjacent cells are held 

 together, is a more difficult question, and one into which I 

 do not propose to enter at the present time. It is enough for 



