1912.] T. gambiense and its Relation to G. palpalis. 529 



the first indication that these factors have begun to come into operation is, 

 most often a reduction in the percentage of dividing forms. The actual 

 disappearance of the trypanosomes is, moreover, generally preceded by a 

 shorter or longer period, during which the numbers are relatively stable 

 and the divisions few. There are, however, occasional exceptions to this 

 rule, and the numbers may suddenly be checked and reduced in the middle 

 of a rise. 



The mechanism by which the trypanosomes are destroyed has not been 

 studied, but the work of other observers all points at present, as far as I 

 am aware, to phagocytosis. One must, however, consider in passing (if 

 only to dismiss it) the possibility of the trypanosomes entering into the 

 cells of the host, and undergoing some form of development at this period. 

 All the evidence is against this assumption on account of the important fact 

 that the trypanosomes reappear relatively gradually, and their appearance 

 is always accompanied by active multiplication in the blood stream. This 

 fact is also of interest in showing that the disappearance is a genuine 

 destruction and not a mere withdrawal of the parasite from the peripheral 

 blood. 



Lysis in the blood stream is also a possible explanation of the method of 

 reduction in the numbers. There is a certain amount of evidence in favour 

 of this to be drawn from a consideration of the endogenous cycle. It will be 

 discussed in a later section. 



The essentials of the fluctuation just sketched are repeated interminably 

 with monotonous similarity, the only features liable to variation being the 

 absolute numbers of trypanosomes involved and the time coefficient. That 

 is to say, the drop may leave a fair number of parasites in the blood, and 

 the rise may occur without further reduction, or the drop may be very 

 complete ; it may be sudden or more gradual. The depressed period may 

 be long or short. Thus also the rise may be more or less rapid; the exalted 

 period may be longer or shorter ; the whole process may take place with 

 low absolute numbers, that is to say, the exalted period may not involve 

 a very high number of parasites — this last is often very characteristic of the 

 later months of an infection. A stable or relatively level period may 

 proceed to a rise instead of to a drop, owing probably to the removal of some 

 temporary inhibition. 



The extreme variability of these two elements, namely, the duration of 

 time of the various periods and the absolute number of trypanosomes involved, 

 produces a quite fictitious appearance of confusion. The essentials of the 

 process are in reality extraordinarily constant. 



As a matter of practical technique these points are more easily demonstrated 



