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The Occurrence of Two Significantly Distinct Races of Endamoeba Histolytica^*

1. A study of the mean diameters and size distributions of cysts of E. histolytica from 99 cases demonstrates the existence of large numbers of strains having mean diameters intermediate between and beyond the 5 races postulated by Dobell and Jepps. These strains overlap greatly in size distribution, do not show size constancy, and differ in no other respect than that of size alone. They, therefore, cannot properly be designated as races of E. histolytica.

2. Further analysis, on the other hand, both of individual strains and the pooled size distribution of 7495 cyst measurements from 320 cases, demonstrates the existence of two significantly distinct races, a large and a small. These are not only distinctly unlike in size, but differ as regards motility, culturability, and pathogenicity in man and lower animals.

3. In practice, the 10 micron line may be generally used to distinguish living cysts of the large or small race. For measurements of wet-fixed stained cysts, mounted in Canada balsam, the 9 micron line serves.

4. The evidence indicates racial constancy both as regards size and physiological characteristics exhibited by the large and small race of E. histolytica.

Received March 14, 1941.
^* Read at the Thirty Seventh Annual Meeting of the American Society of Tropical Medicine, at St. Louis, Mo., November 10–13, 1941.

¹ Statistical analysis by the junior author.