6 TROPICAL MEDICINE AND HYGIENE 



so that the cyst wall is complete. The cyst is discharged 

 and passed with the faeces of the host. Development 

 of the contents takes place, the cell mass divides into 

 four, and in each of these four divisions two spores 

 " sporozoites " are formed. This stage of development 

 takes place in the oocysts as they lie on the ground, no 

 host being necessary in this stage. This is the sexual 

 form of multiplication. Ultimately, when the cyst is 

 swallowed by a suitable host, the capsule is dissolved, the 

 sporozoites are liberated in the alimentary canal and 

 enter cells in the mucosa or pass up the bile ducts in 

 the * liver and there recommence the cycle of events 

 described, multiplying asexually to form massive tumours, 

 or becoming sexual forms, gametocytes, male or female. 



This protozoal infection is common in rabbits ; it has 

 been described in man, but is certainly extremely rare. 

 It forms a good example of asporozoal organism, parasitic 

 during its stages of growth and asexual multiplication, 

 but not throughout the whole of its sexual development 



(fig. i). 



The general plan of development of the parasites of 

 malaria resembles this to some extent, but there are 

 important differences. 



By coccidia no second host is required. Conjugation 

 takes place in the warm-blooded host and full sexual 

 development outside under ordinary conditions, but for 

 many of the protozoa a second host is required and the 

 development in this second or alternate host differs from 

 that in the first host. 



Intermediate and definitive hosts are more precise 

 terms. The definitive host is the host in which the 

 sexual processes of multiplication or reproduction are 

 carried out. In the case of the parasites of malaria the 

 definitive host is the mosquito. 



Intermediate hosts are those in which the asexual 

 method of multiplication is carried out ; e.g., man is the 

 intermediate host of the malaria parasite. 



Insects or other alternate hosts are not required for 



