1912.] T. gambiense and its Relation to G. palpalis. 539 



The above observations upon the production of the resistant trypanosomes 

 are only what one would expect upon the assumption that the Protozoa are 

 capable of evincing reactions corresponding in some sort to the phenomena of 

 immunity in the Metazoa. 



The very first day on which trypanosomes are to be found in the blood of 

 a newly infected monkey falls into exactly the same category as any other 

 active period of multiplication, and is positive or negative to fly according 

 to whether the adult type is present in sufficient numbers or not. 



While in the actual experiments here described the negative periods do not 

 seem to be of very long duration, nevertheless in feeding many cages on a 

 population of monkeys at random these negative states have a profound 

 influence on the results. Thus, in a previous group of experiments, out of 

 62 cages fed only 29 showed infected flies. 



All the above results are, it is clear, entirely opposed to a sex interpretation 

 of the dimorphism. If the long forms were males and essential to the 

 development in the fly then the depressed periods would not be infective. 

 The question of sex does not seem to me to require further discussion in 

 relation to the endogenous cycle. 



VI. General Summary. 



1. T. gambiense is a polymorphic species with a continuous range of 

 variation. 



2. The shorter forms of 14 — 20 n in length constitute the normal adult 

 blood type. 



3. The intermediate individuals measuring 20 — 24 p in length are growth 

 forms and lead to the long forms of 23 — 33 yx, which are those about to divide. 

 The polymorphism of the trypanosome is thus due to growth and division 

 phenomena and does not correspond to a sex differentiation. 



4. The shorter forms are those destined to carry on the cycle in the 

 transmitting host. 



5. There are definite periods when the blood is not infective to fly although 

 trypanosomes are present. Such periods are (a) just before an outburst of 

 multiplication ; (b) during the destruction of trypanosomes preceding a 

 depressed period ; (c) the summit of an exalted period involving very 

 numerous trypanosomes — at such a time the parasites very frequently show 

 signs of exhaustion ; (d) certain periods of rapid multiplication when both 

 the absolute and relative numbers of the shorter forms are low. 



