which are more fixed to the blood and less to the nervous system.
We think it necessary to draw attention to experiments which we have made in connection with the reappearance of trypanosomes after treatment with atoxyl. We found that strains obtained from animals which had had relapses were sometimes much more virulent. Our nagana strain, used for work generally, killed a rat in from live to seven days. This strain was followed up, commencing on January 14th until May 5th, by subinoculation from one rat to another. Four drops of blood were always used. In none of these rats was any sudden increase of virulence noticed; but in experiments with the strain obtained from animals in which the trypanosomes had reappeared it was found that the experimental animal was killed in from three to five days. Out of seventeen rats used for these experiments, eleven showed a remarkable increase of virulence. Similar work done with Trypanosoma dimorphon showed the same increase of virulence. Our strain used to kill a rat in from fifteen to seventeen days, and we found that in four out of six strains obtained through relapses after treatment by atoxyl the virulence obtained was from eleven to nine days. We have recently obtained through atoxyl a Trypanosoma dimorphon strain which kills a rat in seven days. Similar work with Trypanosoma gambiense is now in progress.
It was our intention not to publish the results of this work until we had satisfied ourselves that Trypanosoma gambiense behaves in a similar way, but since it is intended to commence atoxyl treatment on a large scale in Uganda, we feel that a warning in this direction should be given.
These observations emphasise the necessity for careful selection of "fly-free" sites for the separation camps and for some provision by which treated natives may be kept under observation for considerable periods.
