ECOLOGY, COMMENSALISM AND PARASITISM 395 



Between the latter and the membrane there is a delicate linin 

 reticulum. Kofoid and Swezy (1924-1925) describe the division 

 of the endosome after migrating to the periphery of the nucleus, 

 and formation of a spindle figure with a centrodesmose on which 

 6 chromosomes divide. 



Nutrition of the tissue invading form is primarily by endosmosis; 

 erythrocytes are frequently found in the food vacuoles sometimes 

 in large numbers, but just as often there are none at all. Bacteria 

 are present only exceptionally. 



The minuta form is much smaller than the invasive form and 

 there is a greater variation in size— the variations being so consistent 

 that many authors (e. g., Wenyon and O'Connor, 1917; Dobell and 

 Jepps, 1917, 1918) regard them as distinct races. The sizes of the 

 cysts which they form likewise vary. Wenyon (1926) gives the 

 limit of size of the minuta amebae from 7 /x up to the size of the 

 invasive type while the cysts vary in size from 7 to 18 n- 



Reproduction of the ameboid forms by simple division keeps up 

 the number of parasites in the intestine, and may continue indefi- 

 nitely in carriers. Conditions leading to the formation of minuta 

 types and precystic amebae in the intestine are only matters of 

 surmise, but with encystment multiple division into 4 small amebae 

 occurs. The cysts are usually spherical with smooth walls and from 

 5 fx to 20 it in diameter, and when fully developed contain 4 nuclei 

 (in some rare cases 8 may be present, Wenyon). In addition to 

 nuclei so-called chromatoid bodies are present in the cytoplasm. 

 These are of considerable diagnostic value for they are much more 

 rare in cysts of E. coll. They are usually in the form of rods with 

 rounded ends but may be filamentous, or irregularly shaped bodies 

 sometimes 2 or 3 in number, sometimes many. These appear to 

 be absorbed during the external life of the cyst. During the forma- 

 tion of the cyst the glycogen which is present at the commencement 

 of encystment disappears. Cleveland and Sanders have studied 

 ex-cystation. These cysts give rise to an ameba free from chro- 

 matoid bodies and with 4 cystic nuclei. So-called "metacystic 

 development" (Dobell) results in the formation of 8 young, uni- 

 nucleate amebae but not always by the same process. The 4 cystic 

 nuclei may all divide after which the cell divides into 8 uninucleate 

 forms. Or one of the 4 cystic nuclei may divide to form 2 meta- 

 cystic nuclei which with cytoplasmic division gives rise to an 

 ameba with 2 metacystic nuclei, and a sister cell but larger with 

 3 cystic nuclei. In other cases 2 or 3 of the 4 cystic nuclei may 

 divide and be cut off with some cytoplasm which ultimately result 

 in uninucleate forms. Hence metacystic amebae may be found 

 with any number of nuclei, from 1 to 8. Cleveland and Sanders 

 describe 24 such combinations. 



While probable that E. dysenteriae (histolytica) may be carried 



