226 PROTOZOAN PARASITISM 



overlooked unless the lighting is properly adjusted. 

 It is not improbable that they have occurred in feces 

 which have been examined microscopically but have 

 been overlooked. The impenetrability of the cyst 

 makes staining unsatisfactory. In fresh feces it is 

 only the first one or two stages of the oocyst that oc- 

 cur. Stools aged at ordinary temperature develop 

 in from one to three or four days, according to the 

 climate. The number of cysts is small, as a rule, and 

 they persist for only a few days, although Wenyon and 

 O'Connor (1917) observed them during twenty-five 

 days in one case. 



Attempts to infect other animals experimentally 

 have apparently been failures and literally nothing 

 of proven nature, except the oocysts, is known of 

 the organism, its habitat or effects. 



It is reasonably presumed that its habitat is the 

 small intestine, where it passes the phase of schizog- 

 ony in the epithelial cells, and that it is transferred 

 from host to new host by the ingestion of oocysts in 

 fecal contaminated food and drink. Direct transfer, 

 as is most effective in some other intestinal protozoa, 

 seems unlikely, since the most favorable state would 

 be the maturing of the oocyst, with the formation of 

 merozoites, before being swallowed by the host-to-be. 



ISOSPORA HOMINIS 



The infection of the interior of the villi of the 

 mucosa of the small intestine of a man recorded by 

 Virchow (1860) is, according to Wenyon, the only 



