156 SOME MINUTE ANIMAL PARASITES 



unfavourable conditions, while the spore phase also 

 serves as a resting or recuperative period for the 

 organism concerned. E. histolytica also was thought 

 to form spores, and the process strongly resembled 

 that of gemmation, the main difference being that 

 the spore surrounded itself with a thick, yellowish 

 sporocyst, which readily resisted desiccation, and 

 was believed to serve as the starting-point of a new 

 infection after some time had elapsed. These sup- 

 posed spores are small, about half the diameter of a 

 red blood-corpuscle. Some workers believe that 

 these remarkable, small spores are produced by old 

 trophozoites, such as are found during convalescence 

 and after apparent recovery of the victims of amoebic 

 dysentery. On the other hand, many investigators 

 consider that the so-called small spores of E. histoly- 

 tica are of fungoid nature. The correct sporogony 

 of E. histolytica must be sought in the cyst formation 

 of the organism formerly known as E. tetragena 

 (Viereck). Small forms of the latter have been de- 

 scribed from dysenteric cases in South America by 

 Elmassian, and called E. minuta (Fig. 33, C, D). 



Viereck (1907) described an amceba from cases of 

 dysentery, and named the organism Entamceba tetra- 

 gena, as its outstanding feature was the formation of 

 cysts containing four spores. It was the detailed study 

 of E. tetragena that revealed that one organism only 

 was responsible for the diseases referred to three 

 different species of Entamcebae, and that E. histolytica, 

 E. minuta, and E. tetragena were different phases of 

 one and the same vector of disease. The organism, 

 until recently described as E. tetragena, and on which 



