IT may be taken as definitely established that sleeping sickness is due to infection with a trypanosome (Trypanosoma gambiense), and that this trypanosome is conveyed by a tsetse-fly (Glossina palpalis). But if we proceed to analyse and extend this proposition we soon get into difficulties. We do not know for certain whether man is the only “reservoir” of this trypanosome, or whether monkeys and other mammals, especially native dogs, can also harbour it. Should this prove to be so—though the balance of evidence is against the supposition—it must materially affect prophylactic measures. If we consider next the mode by which the trypanosome is conveyed we find ourselves in the midst of the most conflicting evidence. It is still uncertain, whether the transmission is mechanical or whether there is a cycle of development2 of the trypanosome in the fly; facts appear to be all in favour of the first view, analogy all in favour of the latter. Nor is the question a purely academical one, for if the transmission is mechanical, then the flies are no longer infective after the infecting reservoir (man) is removed; if, however, there is a cycle of development, then it remains to be determined how long an infected fly can remain infective after the infecting source is removed.