HOST-PARASITE RELATIONS I INTESTINAL PROTOZOA 



of Isospora parasitic in man ; one is a small species de- 

 scribed by Virchow, which should be known as Isospora 

 hominis, and the second, the larger species discovered 

 in 1915 and called by Dobell (1919b) /. hominis. For 

 this species Wenyon proposes the name Isospora belli. 

 Dobell (1926a), however, still maintains that the correct 

 scientific name of this species is Isospora hominis. Until 

 this controversy is settled it seems best to continue to use 

 the familiar name Isospora hominis for the most common 

 species reported from man. 



Causey (1926) has recently described as Eimeria 

 butkai what he considers to be a new species from man. 

 More convincing evidence is necessary before this species 

 can be accepted. 



Morphology ana nfe-cycle of Isospora hominis. Iso- 

 spora hominis is known only in the oocyst stage (Fig. 

 15). The oocysts pass out of the host in the feces. Their 

 shape is shown in Fig. 15. They measure from 2^(1 to 

 33)U long and from 12.5JU to i6m broad. Usually their 

 protoplasmic contents form a ball; this is protected by 

 the oocyst wall which consists of a thin inner layer and 

 a thicker resistant outer layer. After leaving the body 



Fig. 21. Diagrams illustrating the asexual and sexual cycles of Isospora 

 felts of cats and dogs. Stages 26, 28, and 29 are oocysts which pass out 

 of the body with the feces ; in each oocyst two sporoblasts are formed (28) 

 and in each sporoblast, 4 sporozoites (29). When ingested by a susceptible 

 animal the sporozoites escape from the oocyst (30), enter epithelial cells (i) 

 where they undergo schizogony (2-8). The merozoites produced may 

 repeat the asexual cycle (8 to i to 8) or initiate the sexual cycle (9-25). 

 In the latter, female cells ( 9) or macrogametes (19-22) and male cells (6 ) 

 or microgametes (11-18) develop. Fertilization (23) is followed by the 

 formation of the oocyst (24-26). (Drawn by Dr. Justin Andrews.) 



190 



