The asexual cycle of Plasmodium vivax and Plasmodium falciparum has been cultivated in vitro in human blood. The parasites have been grown also in red blood cells in the presence of Locke's solution, free of calcium chlorid and in the presence of ascitic fluid.

The parasites grow within red blood cells and there is no evidence that they can be grown outside of these cells.

The parasites are destroyed in a very few minutes in vitro by normal human serum or by all modifications of serum that we have tested. This fact, together with numerous observations of parasites in all stages of growth apparently within red cells, renders untenable the idea of extracorpuscular development.

Leucocytes phagocytize and destroy malarial plasmodia growing in vitro only when the parasites escape from their red blood cell capsule or when the latter is perforated or becomes permeable.

Successive generations of Plasmodium vivax and Plasmodium falciparum have been cultivated in vitro by removing the leucocytes from the culture and by transplanting to fresh red blood cells and serum at proper intervals.

The asexual cycle of Plasmodium vivax and Plasmodium falciparum cultivated in vitro does not differ from the same cycle growing in vivo.

The sexual cycle has not been cultivated, though we have obtained some evidence of the possibility of its accomplishment.

There can no longer be any doubt that Plasmodium vivax and Plasmodium falciparum are separate and distinct species. When grown in an identical culture medium and under exactly the same conditions they remain distinct.

In twenty-nine cultures of æstivo-autumnal parasites many forms and sizes have been observed, so that evidence is supplied of the occurrence of different varieties of ætivo-autumnal malarial plasmodia. The so called tertian ætivo-autumnal variety may be seen at the proper stage in all cultures grown from merozoites.

The form and appearance of the same culture of plasmodia may vary greatly under different conditions which are not necessarily destructive to the parasites. Their generation period may vary from thirty hours (ætivo-autumnal) to four days (tertian), as a result of variation in the temperature at which they were cultivated. Sexual parasites grow in the cultures and are more resistant to unfavorable conditions than schizonts, often living several days after the latter die out.

Forms suggesting parthenogenesis have been observed.

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