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If under normal conditions of health and habit the blood be examined during the day, the parasite is rarely seen, or, if it be seen, only one or two specimens at most are encountered in a slide. It will be found, however, that as evening approaches, commencing about five or six o'clock, the microfilariæ begin to enter the peripheral circulation in gradually increasing numbers. The swarm goes on increasing until about midnight, at which time it is no unusual thing to find

Fig. 98.—Minute anatomy of a microfilaria, and differential points of structure between mf. loa and mf. bancrofti. (After Fulleborn.)

N., Nerve ring (ant. break in cell column); A., ant. v-spot (exc. pore); Ex.C., excretory cell; C.G., granular mass (or Innenkörper); G. 1-4, "genital cells"; P., posterior or tail spot; M., granules in mouth cavity, T., granules in tail; Sh., sheath.

as many as three hundred, or even six hundred, in every drop of blood; so that, assuming that the parasites are evenly distributed throughout the circulation, it may be inferred that as many as forty or fifty millions are simultaneously circulating in the blood-vessels. After midnight the numbers begin gradually to decrease; by eight or nine o'clock in the morning the microfilariæ have disappeared from the peripheral