286 BACTEEIOLOGY 



gelatine culture of the cholera bacillus liquefied by its growth. 

 Prom these observations he concludes that Haffkine's vac- 

 cination only affords protection from the intracellular poison, 

 which is the same in all the bacteria tried, but not from 

 the poison generated externally, which differs in all the 

 varieties, and is the cause of the distinctive disease. 



It was, however, pointed out in reply ' that the pro- 

 ducts of gelatine cultures would be more or less special, 

 and that Haffkine's method protects against cholera virus 

 introduced into the alimentary canal ; while the conclusion 

 regarding the identity of the intracellular poisons in all the 

 bacilli named is not regarded as unimpeachable. 



B. Parasitic Protozoa 



Pathogenesis of protozoa, — We have already seen that a 

 parasitic protozoon may originate morbid processes, in the 

 case of the Plasmodium malaria (pp. 169 and 282). There 

 are, however, a number of other protozoa which frequent 

 the bodies of men and animals, and some of these are be- 

 lieved, with greater or less degrees of probability, to be the 

 cause of certain diseases. The best known of these is the 

 Coccidium oviforme, which is important as throwing light 

 upon the habits of other less known protozoa, and more 

 especially as showing that the presence of such an organism 

 may cause proliferation of epithelial cells. 



Coccidium oviforme. — The life-history of this parasite, 

 which causes a very fatal disease in young rabbits, has 

 quite recently been worked out by R. Pfeiffer,^ although its 

 existence was known as far back as 1839. 



When discharged from the affected rabbit in the faeces, 

 it is a firm, translucent, oval cyst measuring about 36 x 22/x, 

 with granular contents. These contract into a ball, and by 



' Brit. Med. Jowni., March 25, 1893, p. 639. 



^ Beitrage zur Protosoen-Forschung, Heft i. Berlin, 1892. 



