528 Miss M. Robertson. Polymorphism of [July 5, 



number of trypanosomes and a rise in the percentage of dividing forms 

 renders such a development very improbable. 



The factors controlling the numbers of flagellates present in the blood at 

 any given time fall naturally into two categories. 



(1) Those conditioned by the vertebrate host. 



(2) Those conditioned by the parasite. 



Phagocytosis, alterations in temperature and the liberation in the serum 

 of protective substances (possibly also any alteration in the viscosity of the 

 blood) constitute the more important reactions on the part of the host likely 

 to influence the condition of the trypanosomes, to act as inhibiting circum- 

 stances to the multiplication and to cause reduction in the number of the 

 parasite. 



The important factors in regard to the trypanosome itself are the self- 

 conditioned processes within the body of the parasite and the production of 

 toxic substances due to its metabolism. 



By self-conditioned processes I mean such manifestations as the inherent 

 capacity for division and nutrition and the general vigour and adaptability 

 of the parasitic organism. The actual reduction in the numbers of the 

 trypanosomes sets in as soon as the nugatory conditions, whatever their 

 origin, get the upper hand. It is obvious that a continual state of tension 

 .exists between the capacity of the host to destroy the parasite and the 

 capacity of the parasite to maintain itself. If these facts are borne in mind 

 the nature and the details of the fluctuations are more easily comprehended, 

 though it may not be clear exactly which set of factors is at work. 



The two sets of factors here mentioned form, it is clear, merely two 

 general groups of conditions ; each group represents obviously a very complex 

 aggregate of interacting circumstances concerning which we have at present 

 practically no knowledge. 



Thus, so far from knowing what inhibits the multiplication of an organism, 

 we have still, in spite of the very valuable work of Hertwig, Loeb, and 

 Klebs, very little knowledge as to what are the conditions that determine 

 the normal division of any cell. This last must be torched upon later. 

 It is not, therefore, to be supposed that the unravelling of the conditions 

 obtaining in an infected monkey does more than afford a certain clearness 

 and precision to our view of the picture presented. 



The drop in the number of trypanosomes is often sudden, but the com- 

 pleteness of the clearance varies within wide limits, and the duration of the 

 depressed period is quite inconstant. 



The complex factors at work producing such a reduction may supervene 

 at any period as regards the absolute number of trypanosomes present, but 



