122 THE BACTERIOPHAGE 



visible in stained preparations, as a bacterium of minute size. 

 All the parasitic ultramicrobes are intracellular parasites, for 

 the lesions which they produce are, in all cases, characterized 

 by protoplasmic inclusions or alterations in the nuclei of the 

 cells. The bacteriophagous ultramicrobe differs from the other 

 known ultramicrobes only in its elective action for unicellular 

 organisms. The others act in multicellular organisms. 



It would be indeed strange that of all living organisms the bac- 

 teria alone should enjoy the privilege of absolute immunity. Such 

 an immunity must have seemed remarkable even before the dis- 

 covery of the bacteriophage, before ultramicrobes sufficiently 

 small to parasitize them had been recognized. The ultramicrobe 

 is in diameter certainly 2000 times smaller than a bacterium of 

 average size, in volume nearly 2000 million times less. In size, 

 one of these ultramicrobes is, to a bacterium, as the bacterium 

 is to a large fly. 



It should be remembered, however, that although up to the 

 present time parasitism of bacteria has not been recognized we 

 have for a long time observed and studied many parasites which 

 incite infectious disease among the protozoa. Several examples 

 will be found cited among the works of Metchnikoff. 5 



It may be well to mention a study of Dangeard 6 entitled "Sur 

 les parasites du noyau et du protoplasma," for the facts disclosed 

 by this investigator offer certain analogies to those presented in 

 the preceding chapters. But there are these differences, namely, 

 the parasite of Dangeard attacks a protozoan, and its dimensions 

 are such that it can be readily observed microscopically and 

 therefore classified. 



The observations of Dangeard deal with an Oomycete, Nucleo- 

 phaga amoebae Dangeard, which parasitizes the nucleus of Amoeba 

 verrucosa Ehr. The Amoeba verrucosa has a large, doubly-con- 

 toured, spherical nucleus, and also a nucleolus, likewise spherical, 

 whose diameter is about two-thirds that of the nucleus. The 

 substance of the nucleolus is very dense and stains with great 



6 Legons sur la pathologie comparee de V inflammation, Paris, 1892, Masson 

 & Cie. L'immunite dans les maladies infectieuses , Paris, 1901, Masson & Cie. 



6 Sur les parasites du noyau et du protoplasma, Le Botaniste, 1894/95, 

 4, 199-248. 



