II 



tain reagents ; the various aniline colors distinguish at 

 once the protoplasm of bacteria from cell-bodies, fibrin 

 threads, fat-granules, crystals ; for every known bac- 

 terium absorbs one or more of the aniline dyes in wa- 

 tery or alcoholic solution. Yet size, shape, motion, 

 and absorption of aniline dyes do not conclusively prove 

 the bacterial nature of the body under examination, 

 since the same phenomena may be exhibited by material 

 forms which are not bacteria. Micrococci cannot thus 

 be individually distinguished from the granules found in 

 the nuclei of many cells, in leucocytes, and floating free 

 in the blood ; rod bacteria are sometimes closely simu- 

 lated in size and shape by fibrin threads and organic 

 crystals. Groups of bacteria, especially of the micrococ- 

 cus tribe, are simulated by nuclear detritus floating in 

 the blood, as pointed out by Riess ; by large granular 

 cells, the " Mastzellen " of Ehrlich, which are found in 

 large numbers in various inflamed tissues, in diphtheria, 

 typhoid fever, elephantiasis Graecorum, for example ; 

 by cross-sections of fibrin threads in blood-vessels ; by- 

 globular masses usually considered to be leucine, which 

 may occur apparently in any tissue, in normal as well as 

 in certain pathological states. It is true that experience 

 teaches one to distinguish these bodies from bacteria, in 

 some cases, by their appearance and reaction to staining 

 agents. Yet absolute certainty can usually be secured, 

 even by the experienced mycologist, only by cultivation 

 outside of the body. In the independent exhibition of 

 reproductive power, by fission or spore formation or both, 

 lies, therefore, the only positive proof that a particle under 

 examination, exhibiting the size, shape, and reaction to 

 staining agents characteristic of a bacterium, is actually 

 one of these lowly organisms, and not an unorganized 

 mass of similar appearance. 



The theoretical considerations end here, but the prac- 

 tical difficulties begin ; for, in order to be sure that an or- 

 ganism which grows in a liquid outside of the body is 

 the same as the particle previously observed within the 

 tissue, we must be assured thai no other organism can 



