MODE OF INFECTION 



133 



but they are eventually passed out with the faeces. Unlike 

 amebse in the vegetative stage, the encysted amebae are resistant 

 to drying and may live for at least a month in dried or partially 

 dried faeces if not exposed to direct sunhght. They are not, how- 



-n. 



Fia. 38. Comparison of Endamoeha histolytica and E. coli. x 1500. A, E. 

 histolytica, vegetative stage; note small indistinct nucleus (n.), clear ectoplasm 

 (ec), ingested red corpuscles (c.) and contracticle vacuole (c. v.). B, E. coli, 

 vegetative stage; note large distinct nucleus (n.), indistinctness of ectoplasm, com- 

 mon absence of ingested food materials and of contracticle vacuole. A', E. histo- 

 lytica, cyst; note small size (10-14 /x), four nuclei (n.), and "chromidial body" 

 (chr.). B', E. coli, cyst; note large size (15-20 fx), and eight nuclei (n.). 



ever, so resistant to drying as are the cysts of many free-living 

 amebae. 



In this condition the amebae may be blown about by the wind, 

 may contaminate garden vegetables where " night-soil " is used 

 as fertilizer, or may be carried on the feet 

 of flies. If by any of these or other means 

 they reach human food or water and thus 

 secure entrance to the digestive tract, the 

 cyst wall is dissolved by the pancreatic juice. 



Fig. 39. Precystic 

 stage of E. histolytica, 

 sometimes mistaken for 

 a distinct species and 



,. T,,i 1 1 ,•• r named £^. wiwM^o. X 750. 



and tour little amebse, each containing one oi (After Woodcock and 

 the daughter nuclei which were formed when Penfoid.) 

 the cyst first developed, are set free in the intestine and begin 

 to grow and multiply. The active vegetative amebae from an 

 acute case of dysentery are destroyed in the stomach if swallowed, 

 and cannot reach their feeding grounds in the large intestine; 



