HOST-PARASITE RELATIONS: INTESTINAL PROTOZOA 



rence. The animal is reported to become spherical and 

 then to secrete a resistant wall about itself. Within this 

 cyst a large number of nuclei (500 to 600) are formed by 

 repeated division of the original nucleus, and each of 

 these nuclei becomes the center of a minute cell or 

 amoebula. These amoebulae break out of the cyst and de- 

 velop into recognizable Amoeba protens in about three 

 weeks. The cysts of E. coli are similarly formed. All food 

 material is first extruded; then a cyst wall is secreted; 

 and finally the nucleus undergoes three successive di- 

 visions resulting in eight daughter nuclei. At this point 

 in the life cycle the cysts are carried out of the body in 

 the feces of the host and no further development occurs 

 unless they are ingested by a proper host and are in this 

 way again brought into a favorable environment. The 

 cysts of Amoeba proteus must likewise encounter a favor- 

 able environment before they will develop normally. The 

 process of excystation has not been satisfactorily worked 

 out in E. coli, but presumably each cyst gives rise to eight 

 small amoebae which, as in the case of the amoebulae of 

 A. proteus, grow into adult amoebae in their natural habi- 

 tat — the contents of the large intestine. In E. coli the cyst 

 wall undoubtedly protects the organisms from injury 

 during their life outside of the body ; one of the functions 

 of the cyst wall of A. proteus is probably also to carry 

 this species unharmed through periods of adverse cir- 

 cumstances. 



The environment of Endamoeba coli within the intes- 

 tine changes from time to time according to the nature of 

 the food taken in ; for example, the intestinal flora may be 

 changed from one consisting almost entirely of acidophil- 

 ic 



