The trypanosome-infected contents of the stomach of a glossina if injected into a susceptible animal do not communicate the infection, although the parasites may be alive at the time of the injection. Possibly the multiplying forms in the stomach are of the same sexual nature as those of malaria in the mosquito gamete forms and therefore incapable of multiplication in a vertebrate host. Koch for T. brucei, and Minchin, Gray and Tulloch for T. gambiense, state positively that the parasites show marked sexual differentiation within the stomach of the insect host.
The discovery by some of the above-mentioned investigators that jungle-caught tse-tse flies harbour a variety of species of trypanosome " wild forms," while only a small proportion of them convey T. gambiense (sometimes as many as a thousand of these insects were required before a successful infection of monkeys was obtained), makes it evident that extreme precision and care must be observed in order to eliminate sources of fallacy. That the tse-tse fly conveys the trypanosome is certain, and Koch's observation that trypanosoma forms may sometimes be observed in the blood-free droplet of clear fluid which can be expressed from the proboscis, as well as the conditions observed in experimenting, indicate that the infection is conveyed by the bite of the fly. But, even assuming that glossina has been proved to be the proper transmitter of T. gambiense, much remains to be done: the history of the trypanosome, from the time of its removal from one vertebrate by the glossina to its introduction into another vertebrate, has not been completed.
Minchin, having discovered encysted developmental forms of trypanosomes in the lower gut of the tse-tse fly, conjectured that the infection might be transmitted by the droppings of the fly when swallowed by man or other vertebrates. Further observation has not supported this conjecture.
Dealing with the entire life-history of T. gambiense, Miss Robertson summarizes her conclusions as follows;
