FILTRABLE MICROORGANISMS 121 



however, the filtrate will produce disease in cattle in the same manner 

 as the imfiltered lymph. It is known that the symptoms produced 

 by the filtrate are caused by a living organism and not by a toxin, 

 because by successive nitrations and inoculations the disease can be 

 transmitted through a long series of animals, thus indicating clearly 

 that there exists in the filtered lymph a living organism which is capable 

 of reproduction. Another proof that the virulence of the filtered lymph 

 is caused by the presence of living corpuscular elements, and that it is 

 not a mere solution of a toxin, is found in the failure of the virus to 

 pass through filters of finer grain than the Berkefeld as, for example, 

 the Kitasato filter. The microorganism of foot-and-mouth disease has 

 not been cultivated nor made visible. Among other diseases produced 

 by filtrable viruses which as yet remain invisible, are hog cholera, 

 rinderpest, swamp fever, fowl plague and South African horse sickness. 



The invisibility of this group of microorganisms may depend upon 

 either their minute size or their peculiar structure. The most powerful 

 microscopes will not enable us to discern with distinctness objects which 

 are less than o.i/j in diameter. We know of bacteria which in size 

 approach this limit quite closely (M. progrediens, 0.15^ in diameter) 

 and there is no reason for believing that the size of organisms is limited 

 by our ability to see them. As already stated, invisibility may also 

 result from a peculiarity of structure, such as complete transparency 

 and failure to stain with the reagents ordinarily used for this purpose. 



The ability of microorganisms to pass through filters is dependent 

 upon a variety of factors. The size and plasticity of the organism, 

 the fineness of the pores, and the thickness of the walls of the filter as 

 well as the conditions under which the filtration is performed, will all 

 influence the result. 



The failure of the filtrable microorganisms to develop under artificial 

 conditions is to be attributed to their strict parasitism and to our in- 

 ability to imitate exactly in the laboratory the conditions which exist 

 in the animal body. The method of Noguchi, referred to above, and 

 which has done so much to advance our knowledge of the filtrable 

 viruses, was first used to cultivate Treponema pallidum. The culture 

 medium is placed in long narrow test tubes and consists of a piece of 

 fresh sterile rabbit's kidney placed in the bottom of the tube over which 

 is poured sterile unheated and unfiltered ascitic fluid or a mixture of 

 ascitic fluid and agar. The surface of this medium is covered with a 



