1. The description of two distinct types of æstivo-autumnal crescentic parasites given by Craig in Osler's Modern Medicine (with difference in extremities, amount and form of pigment, double-outline, etc.), I have been unable to confirm. 2. The crescentic parasite is throughout its entire existence extracellular (i. e., not within the substance of the red corpuscle, but attached to its surface). It develops from the familiar small ring-form parasite, retaining a circular contour until fully matured. 3. The very young ring-form parasite has no capsule; the macrogamete has one during its development and sexual activity; while the microgametocyte has it only during its development. 4. The Sexual Cycle of the crescent may occur in the circulating blood of its human host. This cycle is probably very rapid, for practically the whole cycle can be seen in a few smears taken from one puncture. The microgametocyte, or male, develops a flagellum (microgamete) which, after the parasite has assumed the crescentic phase, leaves the body of the parasite and enters the body of the macrogamete, or female, during its existence as a round body. Fertilization is then accomplished. 5. The microgametocyte, after the escape of the flagellum, shows evidence of degeneration and is probably cadaveric. 6. After fertilization, the macrogamete assumes the crescentic form, reaches the reproductive phase and undergoes sporulation, the product of which appears to be the small ring-form parasite, Renewed infection of the red cells then takes place. 7. That the sexual cycle is not seen oftener in the circulating blood of man may be due to one or more of the following reasons: (a) The process of reproduction may be and probably is very rapid (some forms of protozoa are exceeded in rapidity of reproduction only by bacteria). (b)The cycle may usually take place in some of the internal organs. (c) It may be that the sexual cycle occurs in the peripheral blood only at intervals, when it becomes necessary for the parasite to renew its exhausted vitality. (d) The occasional occurrence of the sexual cycle in the circulating blood may be due to some slight chemical change in the composition of the host's blood serum. 8. I find no good evidence of conjugation-forms in æstivo-autumnal malaria. I found some of the young ring form parasites in close approximation or overlying each other. As the sexual cycle does take place in the blood of man, such a process of conjugation would seem unnecessary, unless it occurred as an atypical process. The fact that the crescent is more resistant to quinine than the other varieties of malarial parasites might be explained by the presence of a capsule which appears to protect the parasite, certainly during the sexual phase and probably during the " resting " period (when not undergoing sexual changes). The youngest form of ring parasite shows no evidence of encapsulation. This may explain the fact that quinine is best administered just before the paroxysm when the parasite is undergoing segmentation.