ENTAMOEBA HISTOLYTICA 45 



which differ in several respects from the tissue-inhabiting forms. The 

 reduction of size is doubtless effected by division, though the process 

 has not yet been observed. The small precystic amoebae were originally 

 described by Elmassian (1909) as a distinct species — E. miniita — and are 

 sometimes still called " minuta " forms in consequence. Their coirect 

 interpretation* we owe to Walker (191 1, 1913), though I had previously 

 observed and described an exactly comparable phenomenon in the 

 development of E. rananuii (Dobell, 1908, 1909). 



The precystic forms of E. histolytica are intermediate in size between 

 the large tissue-forms and the cysts — the smallest of them being, of 

 course, of the same size as the cysts which they form. There is thus a 

 considerable range of variation in their dimensions. Those races which 

 produce cysts of small size have correspondingly small precystic forms, 

 whilst the strains with large cysts have precystic stages of appropriately 

 larger sizes. In all strains, however, the precystic amoebae have the 

 same general structure. (See figs. 77-80, PI. IV.) They are entirely 

 free from all food inclusions — which are eliminated by digestion or 

 excretion before encystation — and are, when alive, sluggish or sessile. 

 Their nuclei have the s:tme general structure as those of the large forms, 

 but the peripheral chromatin is often in a slightly thicker layer, and the 

 karyosome is often slightly larger in proportion, and some.times — though 

 not often — slightly displaced from the centre. There is also, sometimes, 

 a small amount of chromatin in the zone between the karyosome and 

 the nuclear membrane (cf. fig. 2, PI. I). In all these respects the pre- 

 cystic amoebae approximate in structure to E. colt, from the precystic 

 forms of which those of E. histolytica are often difficult and sometimes 

 impossible to distinguish. (Cf. figs. 2 and 13, PI. I.) 



The precystic amoeba comes to rest, becomes rounded, and secretes 

 a cyst wall, thus becoming completely enc3^sted. Encystation occurs 

 only in the bowel — the precystic amoebae which are passed out with the 

 stools being apparently unable to complete their development outside 

 the body. In my experience they invariably die without encysting. 

 The "encystation " of £. histolytica outside the body has recently been 

 described by Yoshida (1918) ; but his figures unmistakably depict a 

 variety of abnormal and degenerate amoebae undergoing fragmentation 

 and other pathological changes. His experiments appear to show, 

 however, that E. histolytica may, under certain conditions, survive — in 

 a more or less degenerate state — outside the body for a considerable 

 time (up to 72 hours). 



Cysts. — The normal development of the cysts of E. histolytica is as 

 follows. The encysted or encysting organism forms, in its cytoplasm, 

 blocks or masses of a highly retractile substance which gives all the 

 reactions of chromatin. 1 call these chromatoid bodies,'] but they have 

 been given various other names (chromidia, crystalloids, inclusions, etc.). 

 Whether they are formed from the chromatin of the nucleus, or are 

 secreted in the cytoplasm, is still uncertain. Their staining reactions 



* Confirmed by Darling, Wenyon, and many other workers soon afterwards. 



t I have used this term for some years, as it seems to me the most suitable. (Cf. 

 Dobell and Jepps, 1917.) It is unnecessary to enter into the prolonged argument which 

 has recently taken place between Chatton (1917, igiSci) and Mathis and Mercier (1917^, 

 1917/) on this subject. It merely shows the lacunae in our knowledge, and the different 

 interpretations which different observers can put upon the same appearances. 



