I AMOEBA 17 



The encysted phase is essentially a resting phase in the life-history 

 during which the living activities are slowed down. Apart from the 

 gradual using up of the reserve food-material — first the glycogen and 

 then the chromatoid substance — the only conspicuous activity is division 

 of the nucleus which takes place normally twice in succession so that 

 the encysted amoeba contains 4 nuclei (Fig. 6, A, 2, m).^ 



The encysted stage serves for the infection of new individuals. The 

 cysts pass away to the exterior." Under conditions of drought the 

 encysted Amoebae very soon die but under cool and moist conditions 

 they retain their vitality for some time^ it may be as long as five weeks 

 although as a rule not longer than a fortnight. If swallowed^ e.g. in 

 drinking water, the cyst is dissolved under the influence of the digestive 

 ferments and the contents set free in the intestine to start a new infection. 

 It does not appear to be definitely established whether the division into 

 four separate uni-nucleate Amoebae takes place before or after being 

 set free from the cyst. 



E. histolytica appears to be widely distributed over the earth's surface 

 as a parasite of man, although its presence is more conspicuous in warm 

 climates. When it gains a footing in the human body it is apt to persist 

 for prolonged periods, probably throughout life, unless special measures 

 are taken to get rid of it. And a point of great practical importance is 

 the fact that only a comparatively small proportion of infected individuals 

 — perhaps under ten per cent — betray their infection by recognizable 

 symptoms of disease : for the others, while not recognizable as invalids, 

 yet serve all the while as animated reservoirs or carriers of the parasite, 

 disseminating it in the encysted phase and in this way spreading infection. 



Apart from the causation of disease it would appear that Amoebae 

 are of practical importance to man in relation to agriculture. Many 

 green plants are dependent upon the existence in the soil of bacteria 

 which prepare for them a supply of nitrates from which they can obtain 

 their supplies of nitrogen. Now these nitrifying bacteria have active 

 enemies in the form of amoeboid and allied creatures whjch creep about 

 in the soil. Sometimes a rich soil becomes " sick " i.e. its fertility 

 becomes greatly diminished although chemical analysis fails to show any 

 evidence of diminished richness. By baking such " sick " soil for a few 

 hours, or by treating it with chloroform vapour it has been found that a 

 marvellous recovery can be induced and it has been suggested that the 



^ It should be noted for purposes of diagnosis that in the case of the harmless 

 B. coli an additional division normally takes place so that there are eight nuclei 

 within the cyst. 



C 



