406 Col. Sir D. Bruce and others. The Development of [July 5, 



development takes place, then the difficulty of observing the phenomenon can 

 be understood. 



Take the following experiments, for example : — 



Table I. — Flies caught in an Infected Area, kept for some days, and then 



fed on Healthy Animals. 



Trypanosoma brucei — Glossina morsitans. 



Expt. 



Place. 



Observer. 



No. of 

 flies fed. 



No. of times 

 flies fed. 



No. of days before 

 infection or under 

 observation. 



Result. 



210 



Zululand 



Bruce 



5 



32 



64 



Negative. 



242 



i) 





30 



11 



56 



JJ 



232a 



>» 



y> 



50 



15 



34 



)) 



These experiments seemed to show that if flies caught in a highly infected 

 district, into which a horse could not be taken even for a few hours without 

 contracting nagana, are kept without food for a few days — say three to five — 

 they are then incapable of conveying infection. This appeared to be a strong 

 proof that the duration of infectivity in the fly was a short one, since, if this 

 were not the case, 1 of the 85 flies ought to have been in a condition capable 

 of infecting, having, of course, been infected at some previous date in the 

 " fly country." It may be repeated, that these flies were caught in a most 

 highly infected district, so that if Glossina morsitans can remain infective for 

 50 or 60 days, 1 at least of the 85 ought to have been in the condition which 

 made it capable of conveying the disease. 



This development of the trypanosomes in the fly is strikingly like what 

 occurs in the test-tube with Novy's medium. A thousand tubes are inoculated 

 with Trypanosoma brucei : the trypanosomes all appear to die off, but 20 days 

 afterwards a peculiarly resistant individual is found in one tube of the 

 thousand, who has adapted himself to the new enviroment, and soon 

 multiplies into myriads. What it is which enables this particular individual 

 to adapt itself to such altered conditions is unknown. It is the merest 

 speculation to call it a sexual act and pick stout forms as females and slender 

 forms as males. 



Again, because this late development of the trypanosomes enables 

 a particular fly to remain infective for 100 days, or even possibly for 

 the remainder of its life, it by no means follows that this is the usual 

 method of infection. The mechanical transference of the disease is proved 

 up to the hilt, and for every case which falls a victim to the rare late-infected 

 fly, a thousand must be infected by direct mechanical transference. 



