70 Notes on the Life-History of Trypanosoma gambiense, etc. 



and activity. Proventricular forms when injected into clean monkeys do not 

 produce infection. 



Invasion of the Salivary Glands. — The long slender forms from the proven - 

 triculus come forward into the hypopharynx in small numbers at a time and 

 may be found lying free in this situation in carefully dissected specimens 

 before the glands are infected. From the hypopharynx they pass back along 

 the narrow ducts of the salivary glands and it is not at all a rare occurrence to 

 find trypanosomes in the ducts of the glands in 1G- to 30-day flies when the 

 rest of the glands show no flagellates at all. The trypanosomes reach tbe 

 glands as long slender forms and attach themselves where the duct joins on 

 to the slightly broader part which leads to the glandular portion proper. 

 They become much shortened and very much broader and assume the 

 crithidial condition shown in figs. 16-21. They break free occasionally but 

 seem to attach themselves again. Multiplication (fig. 22) occurs and the 

 trypanosomes gradually invade the whole gland ; new specimens keep on 

 arriving from the hypopharynx. The short dumpy crithidial forms develop into 

 trypanosomes almost identical with the blood-forms but often a little below 

 the normal adult length (figs. 23 and 24). These trypanosomes are found 

 swimming free in the lumen of the glands and there is the strongest 

 presumptive evidence for considering that these are the types that produce 

 the infection in the vertebrate. 



Not only is this second development in the glands necessary to produce an 

 infective fly, but from a number of considerations, amongst others the 

 appearance here of the very clear and definite crithidial stages, it may be 

 held that the development in the glands is the really essential part of the 

 whole cycle. The development in the gut may be considered as a somewhat 

 indifferent multiplication— a mechanical device to enable the trypanosomes 

 to establish themselves in sufficient numbers in contact with the salivary 

 fluid, which alone, in the Glossina, seems able to stimulate the trypanosomes 

 to the apparently essential reversion to the crithidial type. 



Conjugation. — -Sexual differentiation has not been observed at any part of 

 the cycle ; this is not, however, a characteristic feature of flagellate life- 

 histories. Isogamy seems to be usual among the group. The direct evidence 

 of conjugation is slight and not sufficiently convincing. General theoretical 

 considerations are, however, very strongly in favour of some such process 

 occurring, and from comparative evidence drawn from the consideration of 

 the cycles of T. nanum and T. vivax it seems possible that the sexual part 

 of the cycle might take place in the salivary glands. 



It is obvious that much of the foregoing work has been simply to carry 

 somewhat further the researches of Minchin, Eoubaud, Bruce, and Kleine, 



