132 



AMEBiE 



carried on a long series of experimental feedings with amebse of 

 various species and largely as a result of their work the true facts 

 of the case have been unraveled. They proved the harmlessness 

 of Endamceba coli and also showed that E. histolytica and E. 

 tetragena, long considered distinct species, are really two phases 

 of a single species. 



The Dysentery Ameba. — The dysentery ameba, E. histolytica 

 (Fig. 37), is large and active, 25 to 40 m (tA^ to ^is of an inch) in 



diameter, with a rather trans- 

 parent appearance and blunt 

 pseudopodia. The distinct 

 clear outer layer of protoplasm 

 and very indistinct eccentri- 

 cally placed nucleus, together 

 with the presence in the body 

 of vacuoles and particles of red 

 blood corpuscles in process of 

 digestion, are its distinguishing 

 characteristics. A comparison 

 of the vegetative form with 

 that of E. coli is shown in 

 Fig. 38A and B. 

 There are two stages in the life history of this ameba, the 

 vegetative and the cystic. As long as conditions in the intestine 

 are favorable for their growth and development, the amebse con- 

 tinue in their active vegetative condition, multiplying by simple 

 division of the body into two. When conditions have become 

 unfavorable for them, however, as in later stages of the disease, 

 they decrease in size down to seven or eight ju (about ^i^^jf of an 

 inch) in diameter, become round in form, and begin to develop a 

 tough cyst wall around themselves. This is known as the pre- 

 cystic stage (Fig. 39). From this stage they pass rapidly into 

 the cystic stage by the completion of the cyst wall and the divi- 

 sion of the nucleus into four daughter nuclei, thus forming the 

 well-known " tetragena " cysts (Fig. 38 A'), long supposed to 

 belong to a distinct species. Examined under a microscope they 

 look like tiny globules with a mother-of-pearl reflection. These 

 cysts can readily be distinguished from those of Endamceba coli 

 in that the latter usually have eight nuclei instead of four (Fig. 

 38B'). .The cysts may remain in the intestine for a long time, 



Fig. 37. Endamceba histolytica, living 

 specimen showing ectoplasm and endo- 

 plasm, and several ingested blood corpus- 

 cles. X 1000. 



