• (1)

    A study of the periodicity of microfilarise has been made in monkeys naturally infected by a worm which probably belongs to the genus Dirofilaria. The microfilariae have a nocturnal periodicity similar to that of Wuchereria bancrofti. Subsidiary studies have been made on other similar filarial infections of monkeys and on Dirofilaria repens of dogs.

  • (2)

    By means of an empirical technique, the number of microfilariae counted in a histological section can be used to calculate the total number of microfilariae present in an organ or tissue.

  • (3)

    Complete postmortem examinations have been made on five monkeys and four dogs, half of them killed at midday and half at midnight, respectively. Shortly before and after death (if it occurs in the day) great numbers of microfilariae are liberated from the lungs, so that the blood of the left ventricle contains many more than were present in the circulating blood before the animal was killed. This agonal event accounts for Manson's erroneous suggestion that the microfilariae retired during the day to the heart and great vessels (as well as to the lungs).

  • (4)

    At postmortem examination, the organs of the body (apart from the lungs) contain only small numbers of microfilariae either by day or night. Over 80 per cent. of the total number of microfilariae in the body are usually contained in the lungs and in the blood. There are proportionately more microfilariae in the lungs of animals killed by day than in those killed by night, and conversely for the microfilariae in the blood.

  • (5)

    Pairs of specimens were taken for biopsy by day and by night respectively from most of the organs of the body. The microfilariae present in the liver, spleen, marrow, lymph glands, skin and muscle were relatively few, and often there were more present by night than by day. The lungs contained very many microfilariae at all times. In seven out of eight experiments there were more microfilariae in the lungs by day than there were by night and the accumulation of microfilariae in the lungs during the day was amply large enough to account for the microfilariae which had disappeared from the blood during this period.

  • (6)

    No evidence was obtained that the accumulation of microfilariae in the lung causes significant pathological lesions.

  • (7)

    Direct observations were made on the behaviour of living, microfilariae in the intact circulation of infected cotton-rats and dogs. The microfilariae seemed to pass through the capillaries rapidly and smoothly; and accumulation in the small vessels was not observed except as a terminal event before death of the host.

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