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



(1) Flagellates both may and do conjugate without any external visible 

 difference in the gametes. 



(2) If differentiation of gametes does take place it may not occur until 

 the forms are ingested by the transmitting host. 



The next section, which deals with the infectivity to fly of the blood of 

 these different phases of the endogenous cycle, affords further evidence in 

 regard to the question of sex. 



It may be pointed out in passing that a clear view of the endogenous 

 cycle reveals a wide range of difference in the physiological state of the 

 trypanosomes at different times, and it is clear that this is worthy of 

 consideration from the therapeutical aspect of the trypanosome problem. The 

 introduction of a drug would probably produce somewhat different effects, 

 according to the moment of the cycle chosen for its application. 



V. The Relation of the Endogenous Cycle to the Production of the 

 Infected Fly. 



The chief point brought forward by this aspect of the question may briefly 

 be stated as follows : — 



Is there any definite condition either of the individual parasite or of 

 the infection as a whole requisite for the production of infected glossina ? 



In a previous paper it has been recorded, from experiments treated in a 

 different manner,* that negative periods in relation to fly occurred, although 

 trypanosomes might be present in the blood at the time of feeding. More- 

 over, these experiments pointed towards the trypanosomes rather than the 

 fly as being responsible for the negative result. The present work has 

 confirmed this general observation. 



Failure to infect fly must depend on some one or other of the following 

 factors : — 



(1) Failure of the flies to feed ; this maybe called an adventitious negative, 

 and can be obviated by keeping the flies for at least 24 hours after they have 

 emerged, and by the exercise of patience and observation at the time of feeding. 



(2) Absence of trypanosomes from the blood imbibed by the flies in the 

 experiment. 



(3) Absence of sufficient numbers of a given type (or types) of trypanosome 

 capable of surviving in the fly. 



(4) Presence of the requisite type, but in a physiological state unsuitable 

 for survival in the fly. 



(5) The capacity on the part of all the flies in the experiment of digesting 

 all the trypanosomes imbibed. 



* ' Roy. Soc. Proc.,' B, vol. 85, p. 241, et seq. 



