346 
Races of Entamoeba histolytica 
It is of course possible to regard each race possessing cysts of a given 
size as a distinct variety which merits a distinctive varietal name, 
assigned in accordance with the rules of nomenclature. At present, 
however, it seems to us superfluous to invent such names, since we do not 
know how many different races actually exist, and since for those races 
already studied a simple numerical expression of the size of the cysts 
furnishes an adequate distinguishing mark. 
From a practical point of view it is of course most important that the 
cysts of the various strains of E. histolytica should be recognized and 
accurately distinguished from those of the other amoebic parasites of the 
human bowel. This aspect of the matter has, however, already been con¬ 
sidered by Wenyon and O’Connor (1917) and ourselves (1917), and need 
not be further discussed here. 
As we have already noted more than once, our conclusions in general 
appear to agree very closely with those of Wenyon and O’Connor (1917); 
but this is by no means the case when we come to consider the results of 
some other workers who have published figures and statistics relating to 
the cysts of E. histolytica. Two such publications we shall consider in 
some detail. 
1. Kuenen and Swellengrebel (1913). These authors have stated that 
the cysts of E. histolytica vary in size from 11/x to 19p.. They give a 
curve which possesses a single apex situated at 13/x. With regard to 
these findings we will merely note the following points. First, the form 
of the cuiwe obtained does not differ in any important respect from some 
of our own. Secondly, it seems clear that their figures apply, not to the 
whole species E. histolytica, but only to one of the commoner strains 
producing large cysts—the strains producing small cysts having escaped 
the authors’ notice completely. And thirdly, it is clear that no far- 
reaching conclusions can be drawn from the study of a single strain, and 
of only a hundred cysts belonging to it—the number actually considered 
by these observers. Although at first sight, therefore, these results may 
apjrear very different from our own, we believe that they become easily 
reconcilable with them when the deficiencies just noted are taken into 
account. 
2. Mathis ami Mercier (1916, 1917).—In a series of papers these two 
authors have recently made and repeated a number of statements con¬ 
cerning the diameters of the cysts of E. histolytica] and have moreover 
drawn from their findings certain conclusions concerning the life cycle 
of the parasite. Their conclusions are briefly as follows (see especially 
Mathis and Mercier, 1916 a, p. 980, and 1917 a): 
