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, distinguishable 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 geographical 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 corrosive sublimate, two parts; absolute alcohol, one part; glacial acetic acid, 4–5%.) Cysts fixed in this fluid undergo no measurable shrinkage or swelling in the process.7. The apparent loss of size observed in cysts which have been fixed, stained, and mounted in balsam, is due chiefly to the invisibility of the cyst wall in the mounting medium.8. It follows from what has been said above (5 and 6) that to obtain an approximately true value for the dimensions of the living cysts from measurements made upon those suitably fixed, stained, and mounted in balsam, these measurements should be multiplied by the factor 9. We find no evidence of the existence of a sexual dimorphism in the cysts belonging to a single race, such as has been described by Mathis and Mercier.10. Since cysts of E. histolytica may be found with all diameters from about 5μ. to about 20μ—a range overlapping or covering the dimensions of the cysts of the two common harmless intestinal amoebae of man (E. coli and E. nana) and of other cysts and cyst-like bodies occurring in human faeces—it is of considerable practical importance to recognize and distinguish the diverse races of E. histolytica which differ from one another in the dimensions of the cysts which they produce.
THE LOCATION OF GENETIC FACTORS CONTROLLING A NUMBER OF QUANTITATIVE CHARACTERS IN WHEAT