Abstract. In this note we derive partial differential equations for populations that disperse to avoid crowding, paying particular attention to situations in which the ease of dispersal is not uniform among individuals. We develop equations for the dispersal of a finite number of interacting biological groups and for a single age-structured group, and we give conditions under which the latter equations reduce to the former. In all cases the equations generalize the classical porous flow equation-a degenerate parabolic equation that exhibits a myriad of interesting effects. For the special case of two groups we deduce a simple solution in which the species remain segregated for all time.

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The morphology and behavior of living exoerythrocytic stages of Plasmodium galllnaceum and -P. fallax were studied by the use of tissue cultures, phase contrast microscopy, and time-lapse cinephotomicrography. The morphology of exoerythrocytic stages of these two species was essentially that previously observed in fixed, stained material, with the following exceptions: (1) the presence of a filament on one end of the merozoite, (2) the absence of clefts in the cytoplasm of the large schizonts, and (3) the absence of a vacuole-like space around the parasite.The following behavior was observed either directly or in time-lapse sequences: (1) emergence of merozoites from mature schizonts, (2) progressive motility of free merozoites, (3) entry of merozoites, both actively and passively, into host cells, (4) nuclear division in the parasite, (5) the various stages of schizogony, including final production of merozoites, (6) massive infection of host cells, and (7) phagocytosis of merozoites and attempted phagocytosis of mature schizonts by macrophages.Exoerythrocytic stages of P. fallax differed from those of P. gallinaceum in that the merozoites of the former were (1) somewhat more curved in shape and (2) present in fewer numbers in mature schizonts.The use of tissue culture, phase contrast microscopy, and time-lapse cinephotomicrography promises to solve many of the remaining problems concerning exoerythrocytic stages of malarial parasites and their interrelationships with host cells.

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