I AMOEBA 15 



A. proteus in such characters as size, shape of pseudopodia, and so on, 

 but the only ones that call for special mention are a group of species 

 which have taken to a parasitic existence and are thereforfe of practical 

 interest to medical men. Of these parasitic Amoebae — which are usually 

 now set apart as a separate genus with the name Entamoeba — there are 

 three species which are well-known parasites of man. Two of these 

 appear to do no harm, playing the part of scavengers and devouring 

 bacteria, etc. — E. gingivalis, a small Amoeba, commonly found creeping 

 about in the mouth, especially in and about teeth which are not kept 

 properly cleansed, and E. coli (Fig. 6, B), a sluggishly moving Amoeba 

 common in the large intestine. The third species — E. histolytica 

 (Fig. 6, A) — burrows in the wall of the large intestine, devouring and 

 destroying its cells and causing ulceration. In the great majority of 

 cases this ulceration is not sufficient to produce obvious disease, but in 

 other cases where it goes farther it causes dysentery — "Amoebic" 

 dysentery as it is termed to distinguish it from " bacillary " dysentery 

 caused by bacteria — or, it may be, localized abscesses in the liver or, 

 more rarely, in the lung or brain. 



E. histolytica is a smallish Amoeba, measuring as a rule when rounded 

 about 20 /* to 30 /x in diameter.^ Under normal conditions in the body 

 it has a limax-like form, without clear distinction of ectoplasm from 

 endoplasm, and glides along with great rapidity. If examined on a glass 

 slide outside the body at ordinary room temperature it, on the other 

 hand, remains in one spot, pushing out broad flat pseudopodia of clear 

 transparent ectoplasm (Fig. 6, A, i). The endoplasm of E. histolytica 

 is finely granular in appearance and usually contains ingested food 

 material — ^remains of cells of the intestinal wall and, more especially, 

 red blood-corpuscles (Fig. 6, A, i, e). The presence of these latter is a 

 reliable diagnostic feature distinguishing E. histolytica from the harmless 

 E. coli which feeds mainly on bacteria. 



As was the case with A. proteus, reproduction is carried out normally 

 by a simple process of fission. From time to time, however, the Amoebae 

 leave the intestinal wall and make their way into the cavity of the in- 

 testine as a preliminary to encystment. They are now much smaller 

 in size : their nucleus is relatively larger : the endoplasm is full of 

 vacuoles and is without blood-corpuscles. As the time of encystment 

 approaches, reserve food material is stored up in the form of one or more 



1 The unit of length commonly used in biological science for smaU dimensions is 

 the one-thousandth part of a millimetre, usually designated by the Greek letter ^u. 

 A convenient rough gauge always at hand is afforded by the red blood-corpuscle of 

 man which is a circular disc measuring normally from 7 /x to 8 /i in diameter. 



