448 



PROTOZOOLOGY 



and Meleney (1941) found that bacteria-free cysts usually excyst 

 when suspended in various media with living bacteria and in the 

 absence of bacteria, excystment was observed only in the presence 

 of the reducing agents, cysteine or neutralized thioglycollic acid or 

 under conditions of reduced oxygen tension. According to Dobell, 

 in the process of excystation, a single tetranucleate amoeba emerges 

 from a cyst through a minute pore in the cyst wall. The tetranucleate 

 metacystic amoeba produces a new generation of trophozoites by a 

 diverse series of nuclear and cytoplasmic divisions (Fig. 191) which 

 result in production of eight uninucleate amoebulae. These amoebu- 

 lae are young trophozoites which grow into larger ones. No sexual 

 phenomena have been observed during these changes. It is supposed 

 that when viable cysts reach the lower portion of the small intestine 

 or the colon, the changes stated above take place in the lumen and 

 the young uninucleate amoebulae initiate an infection. 



Q) G 

 I cbeb 0[ 



I I \ I \ / I ^ 



Fig. 191. Diagram showing excystment and a common way by which 

 a metacystic amoeba of Entamoeba histolytica divides into 8 uninucleate 

 amoebulae (Dobell). 



