Certain morphological differences render it possible to recognize duck erythrocytes after introduction into the circulating blood of chick embryos infected with P. lophurae. 4 hours afterward, considerable numbers of merozoites have entered duck erythrocytes, while the parasitemia of the chick itself remains essentially unchanged in degree. By estimating the numbers of potentially invading merozoites from blood films made at the time of introduction of duck cells, it was learned that a relatively constant rate of invasion into duck cells by merozoites was maintained. In counter-distinction there was an ever increasing merozoite death rate in embryos not receiving duck cells concurrent with the increase in numbers of parasites. After the injection into parasitized embryos of duck erythrocytes showing but few parasites, no difference was apparent in the rate of merozoite invasion into the introduced cells and the host cells, respectively; but when the percentage of duck cells was greater, the rate of merozoite penetration diminished to zero. The selective penetration of duck erythrocytes which, under the experimental conditions obtaining, constituted only 30 per cent of the total cell numbers of most, and the inability of the merozoites to move independently, taken together, suggest that the greater susceptibility of the duck erythrocyte may be due to greater numbers of accessible areas on its surface. The decreased susceptibility following parasitization indicates that the presence of the parasite alters the cell in such wise that entry of additional parasites is rendered more difficult.

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