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186 and especially, as proved by Kinghorn and Yorke, that it was transmitted by Glossina morsitans, justifies the conclusion that, if not a distinct species, Trypanosoma rhodesiense, as it has been named, is a very definite variety, and that in practice it must be regarded as such.

In human blood (Fig. 50), T. rhodesiense is morphologically indistinguishable from T. gambiense; but if it is passed through the rat a small but variable proportion of the parasites, especially the stumpy forms, will be seen to have their nuclei located posteriorly to the blepharoplast, that is to say, at the non-



flagellar end of the organism. It also, as occasionally in man, exhibits a marked periodicity, disappearing from the blood-stream for weeks at a time.

Glossina palpalis has not been met with in Rhodesia and Nyasaland; its rôle as regards the trypanosome is filled by Glossina morsitans.

According to Kinghorn and Yorke, the peculiarities in the range, distribution, and fly intermediary of T. rhodesiense receive their explanation in the circumstance that the developmental cycle is dependent on atmospheric temperature. High temperatures (75°-85° F.) favour development in the fly, low temperatures (60°-70° F.) are against it.