ENTAMOEBA HISTOLYTICA 47 



gradually in the course of a few days. Cysts which, when freshly passed, 

 show chromatoid bodies in the majority, at the end of about a week 

 show no chromatoids in the majority. The chromatoids can also be 

 seen to grow smaller and disappear in individual cysts examined from 

 day to day. It thus appears probable that both the glycogen and the 

 chromatoid bodies represent reserve materials of some sort — the former 

 being used up while the cyst is developing, and the latter being absorbed 

 in the mature cyst whilst it is waiting to be ingested by a new host. As 

 a rule the cysts contain no other inclusions than those already noted. 



In size the cysts of E. histolytica are subject to great variation. Their 

 diameters range from about 5 yx as a minimum in the races producing, 

 small cysts, up to about 20 /x as a maximum in races forming large cysts. 

 Each race produces cysts of a constant average size, though showing the 

 usual degree of variation round the mean. The figures on Plates I and 

 IV will give a good idea of the difference in size observable in different 

 races. The cysts shown in figs. 72-74, PI. IV belong to a race with 

 cysts having an average diameter of 6'6/i: those shown in figs. 3-5, 

 PI. I are from a race with cysts of an average diameter of iy$/j,; and. 

 those in figs. 75, 76, PI. IV are from a race forming cysts with an average 

 diameter of 15 /*.* Except for their size, there are no constant morpho- 

 logical differences between the cysts belonging to different strains. 



In form, the cysts of E. histolytica are typically spherical or ovoid,, 

 though they are not as a rule perfectly symmetrical ; but their asymmetry 

 is often extremely slight. 



The cyst wall, as in other Entamoebae, is colourless and perfectly 

 smooth. It is formed of a single layer, so far as I can determine, and 

 measures about o'5/i in thicknessf in cysts of medium size (ca. 12 /i); 

 but it is slightly thicker in larger and correspondingly thinner in smaller- 

 cysts. 



The specific gravity of the cysts of E. histolytica is about 1065, 

 according to Ujihara (1914), who also found that the cyst wall is almost 

 insoluble in gastric juice, readily soluble in trypsin, slightly soluble in 

 bile, but resistant to lipoid solvents, such as sodium taurocholate and 

 saponin. Its exact chemical composition is still uncertain. 



The cysts of E. histolytica will survive for several weeks outside the 

 body of man, if they are kept moist and cool. They will live in damp 

 faeces or in water without showing any conspicuous change save the 

 loss of their chromatoid bodies. As a rule, if the cysts are kept under 

 observation, it will be found that some of them remain alive much 

 longer than the others. In water or faeces some will usually be found 

 dead at the end of a week, many more after the lapse of a fortnight, and 

 after this period only isolated survivors will be discoverable. The 

 longest time of survival which I have observed is five weeks (cysts 

 kept in water), but as a rule they will not live so long. Desiccation 

 kills them immediately, and they degenerate much more rapidly at a high 



* These figures are the average sizes of the living cysts. In stained specimens the 

 cysts show an apparent reduction in diameter of about 10 per cent., as I have showrv 

 elsewhere in a joint paper (Dobell and Jepps, 1918). 



t Some observers (Kuenen and Swellengrebel (1913), Woodcock and Penfold (1916)^ 

 Brug (igi/iJ), etc.), state that the cyst wall has only a "single contour" when seen 

 under the microscope. It has only one layer, it is true ; but in optical section both the 

 inner and the outer surface can be seen and the distance between them measured. The 

 " single contour" appears to be due to an incorrect adjustment of the microscope. 



