536 FAMILY: TRYPANOSOMID^. 



2. Trypanosomes may not entirely disappear with digestion of the 

 first feed of blood, but do so at the second. 



3. They may survive and multij)ly in the gut, although a second feed 

 of blood has been superadded. 



i. They may survive and multiply in the crop for as long as twelve 

 days, provided the crop has never been entirely emptied of blood. In such 

 cases the gut may be entirely free from trypanosomes. Those in the crop 

 are unable to survive a complete emptying of the organ, and no permanent 

 infection of the fly results if this takes place. 



5, The trypanosomes may persist in greater or less numbers both in 

 the gut and in the crop of the same fly. 



6. The whole of the partially digested blood which survives from the 

 first feed may be displaced by the fresh blood of the second feed without 

 the trypanosomes which are present in the stomach disappearing. The 

 crop in these cases may be either empty or filled with new blood. 



Of these various conditions, any one of which may result from a feed 

 of infected blood, the last appears to be the one which leads to a permanent 

 infection of the fly. It is thus evident that only a small percentage of the 

 flies actually fed will acquire an infection. In the flies in which infection 

 will occur, active multiplication of the trypanosomes in the mid- and hind- 

 gut commences soon after the infecting feed, and continues progressively. 

 There is no intracellular stage of the trypanosomes, no stage in which they 

 are attached to the gut wall, and in no case do they disappear from the 

 gut to reappear at a later period. Reproducing trypanosomes are thus 

 constantly present in the lumen of the gut. 



Thirty-six to forty-eight hours after ingestion many degenerating 

 trypanosomes are present, together with dividing healthy forms, which 

 appear to be all of the short broad type which were present in the blood of 

 the vertebrate. They differ little at this stage from the blood type 

 (Fig. 224, 1-2), though the undulating membrane may be a little straighter 

 and the kinetoplast slightly displaced towards the nucleus (Fig. 224, 3). 

 The division results in the formation from the parents of daughter indi- 

 viduals, which exceptionally may have, for a short time only, the crithidia 

 arrangement of nucleus and kinetoplast. The trypanosome arrangement, 

 however, is quickly regained. These crithidia forms only occur at the 

 early divisions, and they are the only indication of a crithidia phase in the 

 intestinal development. By the tenth day a large number of trypano- 

 somes remarkable for the variety of their shape and size has been pro- 

 duced, but the maximum length attained by any one does not exceed 

 35 microns (Fig. 224, 4-1 1). At this stage there may app r a small 

 number of characteristic slender individuals. From the tentL to the 

 fifteenth day these slender forms are developed gradually from the broader 



