AMOEBINA 355 



the amoeba may invade the liver in which it produces abscess, or 

 other organs such as lung, brain, testis, etc. The infection in these 

 organs is referred to as amoehiasis. 



Under certain circumstances not well understood, the amoebae 

 remain small after division. Such amoebae are sluggish and known 

 as the precystic forms. The precystic amoeba secretes presently a 

 resistant wall and becomes encysted. The highly refractile cyst is 

 spherical and measures 5-20(j, in diameter. At first it contains a single 

 nucleus which divides twice. The mature cyst contains four nuclei. 

 In addition the cyst contains diffused glycogen and elongated refrac- 

 tile rod-like bodies with rounded extremities which stain deeply 

 with haematoxylin (hence called chromatoid bodies). These inclusions 

 are absorbed and disappear as the cyst matures. No further changes 

 take place in the cyst as long as it remains outside the host's intes- 

 tine. The trophozoites are found in dysenteric or diarrhoeic faeces, 

 but formed faeces usually contain cysts only. 



The life-cycle of Entamoeba histolytica in human host is unknown. 

 The amoeba has, however, been cultivated in vitro by numerous 

 investigators since the first successful cultivation by Boeck and 

 Drbohlav (1924) (p. 717). The excystment of cysts and metacystic 

 development have also been observed and studied especiallj'- by 

 Dobell (1928) and Cleveland and Sanders (1930) in cultures, Snyder 

 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 onl}^ in the presence 

 of the reducing agents, cysteine or neutralized thioglycoUic 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 

 complicated series of nuclear and cytoplasmic divisions (Fig. 168) 

 which result in production of eight uninucleate amoebulae. These 

 amoebulae 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 enter the intestinal 

 wall, bringing about an infection. 



While the description of Entamoeba histolytica given above apply 

 in general, diversities in dimensions of trophozoites and cysts, and 

 in pathogenicity in human host as well as in experimental animals 

 have been reported. A number of observers are inclined to think 



