20 PROTOZOAN PARASITISM 



unsuccessful. In the light of the report of Yorke and 

 Adams (1926), and that of Allen (1926), it is be- 

 lieved that observation of excystation of these am- 

 oebae was truly observed and that primary cultivation 

 was successful. The illustration of the ruptured 

 cysts by Yorke and Adams conforms essentially to 

 those seen and drawn at that time. These writers 

 believe, from their investigations, that excystation 

 of Endamoeba histolytica requires only moisture and 

 suitable temperature (37°C.), while Allen reports 

 that in order for Councihnania lafleuri to excyst in 

 culture the cysts must reach the proper phase and 

 the medium must have necessary chemical and physi- 

 cal conditions, it occurring in their experiments in 

 Ringer's solution with 0.01% dextrine but not in 

 other media. 



At any rate escape from the cysts takes place under 

 conditions suitable for the free activity of the organ- 

 ism and it emerges as one or more, in the different 

 forms. 



REPRODUCTION 



Although reproduction of protozoa may be sexu- 

 ally or asexually, the latter is the usual means in the 

 species parasitic in the alimentary tract of man. 

 Division occurs by binary fission as the means of 

 multiplication in the vegetative phase, two identical 

 bodies, but commonly smaller, resulting. 



The nucleus first divides bv mitosis and then the 



