Page:Tropical Diseases.djvu/193

X] but there is direct evidence that it may continue for several—three or four—years. From what we know of the incubation period of sleeping sickness it is not improbable that this period is sometimes greatly exceeded, and may extend to seven years or longer.

Extrusion of granules and their further development.—Fry and Ranken have recently described under this title a remarkable phenomenon which may have an important bearing on the life-history

Fig. 45.—Trypanosomes shedding granules. (After Fry and Ranken, "Proc. Roy. Soc.")

of the trypanosome (Fig. 45). Balfour describes a similar process in spirochætosis. It is especially observable in those more chronic forms of trypanosomiasis of man and animals, in which the trypanosomes disappear from the blood for considerable periods at a time, and may explain the occasional infectivity of blood in which the parasite cannot be detected with the microscope.

The observers referred to state that trypanosomes may or may not contain two kinds of granules, one kind being of the nature of food material, the