C. Dobell and M. W. Jepps 
849 
part we can only suppose, in the light of our own results, that the con¬ 
clusions of Mathis and Mercier rest upon some fallacy in observation or 
interpretation which, in the absence of all data for its determination, we 
are at the moment unable to define. 
Although we believe that the conclusion of Mathis and Mercier that 
the cysts of E. histolytica are constantly differentiated into “microcysts” 
and “macrocysts,” destined to liberate microgametes and macrogametes 
respectively, is unsupported by facts, we would not be understood to 
maintain, either that the small amoebae which emerge from the cysts 
are not gametes, or, if they are, that they do not belong to two different 
categories. In the absence of any evidence save a somewhat vague 
analogy with other organisms, we think it is as yet unprofitable to discuss 
such hypotheses further. We may, however, point out that, far from 
denying a sexual development in E. histolytica at the stage in the life- 
history and of the kind postulated above, this very suggestion was, we 
believe, made originally by one of us (C. D., 1909) as a result of a detailed 
study of the very closely related sjDecies E. ranarum. 
Summary and chief conclusions. 
1. Entamoeba histolytica Schaudinn (vel E. dysenteriae Councilman 
and Lafleur), the human dysentery amoeba, is a collective species. It 
comprises a number of distinct races, strains, or pure lines, distinguish¬ 
able from one another by the size of the cysts which they produce. 
2. How many such distinct races exist is still undetermined; but we 
have demonstrated the existence of at least five. 
3. There is no evidence that the different races differ in their geo¬ 
graphical distribution, or in any character save size. 
4. These races remain constant in character within a given host; 
and the dimensions of the cysts are not determined by the action of the 
host upon the parasite, since two different races may coexist side by 
side in the same host. 
5. Different values for the dimensions of the cysts are obtained when 
these are measured in different media. Taking the measurements of 
living cysts suspended in physiological saline solution as representing 
the true value, we find that the dimensions of cysts in iodine solution 
are approximately the same; but that cysts which have been suitably 
fixed, stained, and mounted in balsam, have undergone an apparent 
reduction of approximately 10 % from the true diameter. 
6. The most suitable fixative for E. histolytica cysts we have found to 
be a modification of Schaudinn’s fluid. (Saturated aqueous solution of 
