XVIII] LIFE-CYCLE 313 



Individual strains of T. gambiense vary greatly in the percent- 

 age of infected flies they produce, this being due to the greater 

 or less vigour of the trypanosomes interacting with the inhibit- 

 ing forces of the fly. The negative blood periods, during which 

 the parasite is not in a fit state to carry on the cycle, are as 

 well marked in very vigorous strains as in those of lesser 

 vitality. 



When the conditions are favourable to development in the 

 fly, the trypanosomes first become established in the posterior 

 region of the mid-gut. Here multiplication takes place and 

 trypanosomes of very varying sizes are produced, though the 

 parasites rarely surpass a length of 34 to 35 microns. 



About the tenth day numerous trypanosomes are present, 

 and the characteristic slender forms may begin to appear, but 

 only in small numbers. 



The method of division in these forms is essentially similar 

 to that in the circulating blood, but the final division of the 

 protoplasmic body is very characteristic. Instead of the two 

 daughter individuals swinging out so as to be arranged kineto- 

 nucleus to kinetonucleus as in the majority of trypanosome 

 divisions, there is no .longitudinal splitting of the parent organ- 

 ism, but the young specimen is pushed off at the posterior end 

 (Fig. 75, 17) and division is practically transverse. 



After the loth to the I5th day the slender forms that 

 constitute the proventricular type gradually develop from the 

 broader forms, and as their numbers increase move forward into 

 the pro vent riculus, where they are the dominant type. There 

 is only one important point in which they differ from their 

 predecessors. The body is long and slender, the cytoplasm 

 finely granular and much less dense than in the broader forms, 

 but the trophonucleus shews a distinct change. The karyo- 

 some has become much smaller and the membrane has become 

 much more marked and stains deeply. In the fully developed 

 slender types, division rarely seems to occur. 



The infection grows forward by sheer force of multiplication 

 until it fills the whole of the middle and hinder intestine and 

 the posterior part of the anterior intestine. The anterior 

 portion of the anterior intestine and the proventriculus shew 



