blood for the day. This nocturnal periodicity is, under normal conditions, maintained with the utmost regularity for years. Should, however, as Mackenzie has shown, a filarial subject be made to sleep during the day and remain awake at night, the periodicity is reversed; that is to say, the parasites come into the blood during the day and disappear from it during the night. It cannot be the sleeping state, as some have conjectured, that brings about this periodicity; for the ingress of the microfilariæ into the peripheral blood commences three or four hours before the usual time for sleep, and the egress several hours before sleep is concluded, and this egress is not
Fig. 99.—Structure of head end of mf. perstans (a, b), and of mf. bancrofti (c, d).
complete until several hours after the usual time of waking. This night swarming of the larvæ of F. bancrofti in the peripheral circulation seems to be an adaptation correlated to the life-habits of its liberating agent, the mosquito, Culex fatigans, its usual intermediary host.
Some years ago I had an opportunity of ascertaining that during their diurnal temporary absence from the peripheral circulation the microfilariæ retire principally to the larger arteries and to the lungs, where, during the day, they may be found in enormous numbers.
