214 FAMILY: AMCEBID^ 



The nucleus of E. coli is a larger and coarser structure than that of 

 E. histolytica, and is readily distinguished in the living amoeba on account 

 of the low refractive index of the cytoplasm. In stained specimens it is 

 seen to have a thicker membrane than the nucleus of E. histolytica. The 

 chromatin granules are coarser and the karyosome, when it is a single 

 compact granule, is larger, as also is the clear area around the karyosome. 

 Dobell (1919) states that the karyosome is nearly always excentric, and that 

 chromatin granules occur on the linin network between the clear area and 

 the nuclear membrane. The nucleus of E. coli thus differs from that of 

 E. histolytica chiefly in its coarseness, and as the nucleus of E. histolytica 

 quickly changes in character as a result of degeneration, it is very frequently 

 impossible to distinguish the two amoebae as they occur in the stool from 

 the appearance of their nuclei alone. The presence of a larger number of 

 food vacuoles containing bacteria and other objects is a more reliable means 

 of recognizing E. coli. It must be admitted, however, that it is very often 

 impossible to distinguish between E. coli and E. histolytica in the free con- 

 dition. In such cases search must be made for the characteristic cysts. 

 E. coli reproduces by binary fission, like E. histolytica. The details of 

 nuclear division have not been followed completely in the free forms; they 

 are very similar to those of E. histolytica. During the division of nuclei in 

 the cysts Swezy (1922) states that there are probably six chromosomes. 

 Several observers, including Schaudinn (1903), Casagrandi and Barbagallo 

 (1897), and Mathis and Mercier (1917), have described a process of schizo- 

 gony of E. coli. In stained films it is often very difficult to detect the 

 wall of a cyst, which becomes highly transparent in cleared preparations. 

 If such a cyst has an irregular shape, as is not infrequent in prepara- 

 tions, the appearance of an amoeba with eight nuclei is produced. 

 The writer has seen and marked such forms as possible schizogony or 

 multinucleate stages, but in all cases it has appeared more probable that 

 they were distorted or irregularly shaped forms which were really encysted. 

 There seems no reason to suppose that E. coli in the free condition repro- 

 duces in any other way than by binary fission. 



2. Precystic Forms. — As in the case of E. histolytica, prior to encyst- 

 ment there are produced amoebae which are smaller than the adult forms 

 and have a cytoplasm cleared of all food materials (Fig. 100, ii). The 

 precystic forms of E. coli are very similar to those of E. histolytica, but 

 as the average size of the cyst of E. coli is greater than that of E. histo- 

 lytica, so the precystic amoebae are correspondingly larger. These pre- 

 cystic forms are probably formed by division of the larger individuals. 



3. Cyst. — A cyst wall is secreted round a precystic amoeba which has 

 become spherical. The nucleus divides to form two nuclei, these divide 

 to form four, and the four divide again to give the eight nuclei charac- 



