16 PROTOZOAN PARASITISM 



obtaining complement fixation between antigen made 

 of cultures of Endamoeba histolytica and the blood of 

 hosts of this parasite, by Wagener (1924), who found 

 positive precipitation of extracts of scraped mucosa of 

 amoebic colitis by blood serum of cats experimentally 

 infected over a week with Endamoeba histolytica and 

 negative controls in other cats, and by Wagener and 

 Thomson (1924), w^ho found that kittens previously 

 infected with Endamoeba histolytica from chronic 

 human amoebiasis w^ere more difficult to infect with 

 amoebae of more virulent strains than were normal 

 animals of the same age and size. It is, perhaps, in- 

 dicated by the experimental production by the author 

 of encystment of Trichomonas caviae in the blood 

 serum of its own individual host. 



This occurred during the course of culture of 

 Trichomonas caviae at 37.5°C. incubation in medium 

 composed of one part of the animal's own fresh 

 blood serum in nine parts of 0.9% sodium chloride 

 solution, but did not occur in similar medium of 

 other serum. 



It is common knowledge that the number of para- 

 sitic protozoa to be found varies widely from time 

 to time and that they may die out completely. This, 

 however, may be due to alteration of the state or 

 content of the medium in which they live and not to 

 the development of ^'antibodies" by the host. The 

 work of Hegner (1923), in attempting to control the 

 number of flagellates in the intestine of the rat by 

 a protein diet, indicates that the diet may be an im- 



