212 FAMILY: AMCEBID^ 



lytica it lives in the large intestine, but it does not invade the tissues. 

 It develops in the intestinal contents, especially on the surface of the 

 mucosa, where it feeds on bacteria, yeasts, and other material. It will 

 ingest cysts of other Protozoa, such as those of Giardia and Isosiwra, 

 and even the cysts of E. histolytica (Fig. 99). It does not appear to ingest 

 red blood-corpuscles in its natural habitat. In cases of bacillary dysen- 

 tery, when enormous numbers of red cells occur in the stool, E. coli may 

 sometimes be seen moving about amongst them, and showing no inclina- 

 tion to take them in. The writer has seen red blood-corpuscles adhering 

 to the surface of motile E. coli, which, however, showed no tendency to 

 engulf them. Lynch (1924) has, however, been able to induce E. coli 

 to ingest red cells by incubating them with blood in a test-tube. In the 

 writer's experience this never occurs in the intestine, and, if it does, it 

 must be such a rare phenomenon that the general rule given above, that 

 an amoeba with included red cells is almost certainly E. histolytica, still 

 holds for all practical purposes. Like E. histolytica, E. coli becomes 

 encysted in transparent resistant cysts, and it is these forms which 

 spread infection from one individual to another. 



MORPHOLOGY. — E. coli may be considered in three stages: the adult 

 form, the precystic form, and the cyst. 



1. Adult Form. — The fully-groAvn E. coli (Fig. 100) is on an average 

 larger than E. histolytica, and as usually seen it has a diameter of 15 to 

 30 microns. Occasionally very much smaller forms, under 10 microns 

 in diameter, occur. Generally, the amoebae are much less active than 

 E. histolytica, the movements being very sluggish. Occasionally, however, 

 the writer has seen undoubted forms of E. coli moving with a rapidity 

 comparable with that of E. histolytica. The ectoplasm is not so clearly 

 defined as in E. histolytica, and in the normal individual there is merely 

 a superficial layer which is clearer than the endoplasm into which it 

 merges. The degenerating forms of E. coli do not show the exaggerated 

 extension of ectoplasm which is such a characteristic feature of the 

 abnormal forms of E. histolytica. The endoplasm of E. coli is often 

 extensively vacuolated, and the vacuoles contain a great variety of 

 objects which are chiefly bacteria. The general appearance of the amoeba 

 is that of a slightly greyish object, which contrasts with the greenish tint 

 resulting from the high refractive index of the denser E. histolytica. 

 E. coli is much more fluid in consistency than E. histolytica. Sometimes 

 the amoebae show various fissures or rectangular vacuoles, which are 

 probably the result of degenerative changes. 



1-3. Forms with vacuolated cytoplasm, including bacteria. 4. Binucleated form. 



5-6. Forms with irregularly shaped nuclei and very coarse chromatin masses. 



7. Form which has ingested a small binucleated cyst. 



8. Form showing excentric position of karyosome. 



9-10. Small individuals. 11. Large precystic form with clear cytoplasm. 



