MODE OF INFECTION 



133 



but they are eventually passed out with the feces. Unhke 

 amebae in the vegetative stage, the encysted amebs are resistant 

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

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



FlQ. 38. Comparison of Endamteba histolytica and E. coli. x 1500. A, E. 

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

 (ec), ingested red corpuscles (c.) and contraoticle 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 contractiole vacuole. A', E. histo- 

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

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



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

 amebse. 



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 ^,™j^ ^. 



of flies. If by any of these or other means ^^ ^P 



they reach human food or water and thus 

 secure entrance to the digestive tract, the 



Fig. 39. Precystic 

 stage of E. histolytica, 

 sometimes mistaken for 



cyst wall is dissolved by the pancreatic juice, a distinct species and 



,. I'j.i 1 1 j_ • ■ r named E.minuta. X 7 50. 



and four little amebae, each containing one of (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; 



