AMOEBINA 315 



oles contain erythrocytes, tissue cells, leucocytes, etc.; stained 

 nucleus shows a membrane, peripheral chromatin granules, a 

 centrally located small endosome, and indistinct network with a 

 few scattered chromatin granules. The trophozoites invade the 

 tissues of the gut-wall of man and multiply by binary fission. 

 Under certain circumstances not well understood, the amoeba 

 extrudes its food material and becomes smaller in size, possibly 

 by division also. Such a form is sluggish and shows frequently 

 glycogen bodies and elongated refractile bodies which stain deep- 

 ly with a nuclear stain (hence called chromatoid bodies). This 

 stage is known as the precystic stage. The cyst is formed when 

 the precystic form ceases to move about and becomes surrounded 

 by a definite cyst-membrane which it secretes. The cysts measure 

 5-20;u in diameter. At first it contains a single nucleus which 

 divides twice and tetranucleate cyst is thus formed. The glycogen 

 and chromatoid bodies become absorbed, as the cyst grows older. 

 The change between the cyst and the young trophozoite is not 

 definitely known, although in recent years several investigators 

 such as Dobell, Cleveland and Sanders and others, have been able 

 to cultivate the amoeba in vitro and noted the excystment fol- 

 lowed by division into up to 8 uninucleate amoebulae, each of 

 which grows into a mature trophozoite. There is no evidence that 

 sexual reproduction occurs in its development. 



This amoeba was first definitely recognized by Losch in Russia 

 in 1875. It is now known to have a wide geographical distribution. 

 The incidence of infection in man depends mainly upon the 

 sanitary conditions of the community, since the amoeba is car- 

 ried from man to man in the encysted stage. Faecal examinations 

 which have been carried on by numerous investigators in different 

 parts of the world, reveal that the incidence of the infection runs 

 as high as 50 per cent. In the United States 49,336 examinations 

 conducted in various localities show infection rate varied from 0.2 

 -53 per cent, averaging 11.6 per cent, which justifies Craig's 

 (1926) early estimate that ten per cent of the general population 

 harbor this organism. An acute infection by Entamoeba histolyt- 

 ica is accompanied by dysentery, while in chronic cases, the host 

 may void a number of infective cysts without suffering himself. 

 Such a person is known as a 'carrier.' The amoeba invades the 

 liver also and produces in it various abscesses of a serious nature. 

 Numerous varieties are known. Cats and dogs are easily infected 



