PROTOZOA 199 



dogs produce dysentery and liver abscess. In man, 50 per 

 cent, of human beings harbor non-pathogenic amebae, but 

 the pathogenic variety is found mainly in tropical countries, 

 where it produces serious lesions and often occurs in wide- 

 spread epidemics. 



Source. It is supposed to come from poor water supplies. 

 Amebic dysentery differs from the bacillary form in that no 

 severe toxic symptoms are present and the amebic disease 

 is more chronic. The Shiga bacillus, B. dysenteriae, is found 

 in the bacillary form of dysentery. 



Life Cycle of the Malarial Sporozoa. According to its 

 situation, the parasite exhibits two distinct phases of exist- 

 ence : in the human blood it passes through an asexual repro- 

 ductive cycle, known as schizogony, while in the body of the 

 mosquito it undergoes an entirely different series of sexually 

 reproductive changes, called sporogony. 



i. The Asexual Cycle in Man. An infected mosquito con- 

 veys the parasites into the blood of man as minute hyaline 

 bodies which enter the blood-cells. At first they are small, 

 round, colorless bodies, exhibiting more or less active ameboid 

 motion in the fresh blood. Sometimes, particularly in the 

 estivo-autumnal form, a ring shape is assumed. Their size 

 gradually increases and pigment-granules appear, while in 

 stained specimens a nucleus containing chromatin granules 

 is visible. As the parasite approaches maturity the chroma- 

 tin becomes scattered, and finally the protoplasm or mother- 

 cell, known as sporocyte, divides into six to twenty spores, 

 daughter-cells or merozoiies, each containing a portion of the 

 chromatin. The number of spores formed and their arrange- 

 ment before segmentation takes place differ in the three 

 varieties and will be noted below. The spores burst through 

 the envelop of the red corpuscle and become free in the blood, 

 but speedily enter fresh corpuscles and pass through the 

 same series of changes. The febrile stage is synchronous 

 with sporulation and liberation of the young forms. 



Certain of the parasites do not, however, go on to segmenta- 

 tion, but after reaching maturity, remain quiescent and form 



