298 THE PATHOGENIC RHIZOPODA 



membrane disappears, and in its place is secreted a thin but much more 

 refractive membrane, the definitive cyst membrane. Tlie contents of 

 the cyst become again closely united, and the tv^o remaining nuclei 

 are brought closely together. Then follows a third division by mitosis, 

 characterized by long connecting strands which lie parallel with one 

 another in the centre of the cell, so that the daughter nuclei of the two 

 parent nuclei lie side by side in pairs. These nuclei then fuse, an 

 eighth part of one of the original nuclei uniting with an eighth part of 

 the other, while the outer membrane hardens and thickens. Each 

 cyst thus contains two fertilized nuclei, the process recalling the 

 phenomenon in paramecium where, from the same primary nucleus, a 

 wandering and a stationary nucleus is formed. In the fertilized 

 Entameba coli each of the two nuclei divides, forming four nuclei; 

 then each of these divides again, making eight nuclei in the cyst, and 

 in this condition the encysted parasite passes into the intestine of a 

 new host, where the protoplasm of the cell divides into eight parts 

 around the eight nuclei, the cyst membrane is dissolved off and eight 

 small amebse start a new infection with a new potential of vitality. 



This complicated life history has been confirmed in part by other 

 observers, Wenyon ('07) and Craig following out the sexual history 

 in E. muris and E. coli respectively (see p. 142). The possibility of 

 union of two ameb«3e before encystment is not excluded, nor is the 

 possibility of pseudoconjugation, as seen in the gregarines, beyond 

 question. Autoconjugation, while recognized in many different kinds 

 of animals, is too unusual to be granted without the surest proof, 

 and further research on the life history of these parasites is urgently 

 needed. 



The structure of Entameba histolytica, according to Schaudinn, is 

 somewhat different from that of E. coli, and makes it better adapted 

 for its cell destroying function. This is shown by its definite cortical 

 plasm, a layer of firm protoplasm with distinctly higher refractive 

 index than the internal protoplasm, which gives a more rigid character 

 to the pseudopodia, by which the organism is able to force its way 

 between the epithelial cells of the intestine and into the more deeply 

 lying tissues. Schaudinn has watched the organism thus make its 

 way into the epithelial tissue of a freshly extirpated, infected cat 

 intestine, its active movements often lasting an hour, while its own 

 body assumed the greatest variety of forms. The nucleus is difficult 

 to see during life of the organism, a feature in marked contrast to the 

 nucleus of Entameba coli, which Schaudinn recommends as a par- 

 ticularly favorable object for the study of the changes of the living 

 nucleus. The nucleus of ^. histolytica has very little chromatin matter 

 as compared with the nucleus of the other species, but there is a single 

 central karyosome and a slight collection of chromatin around the 

 periphery. While the nucleus of Entameba coli is only slightly vari- 



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