304 THE PROTOZOA 



In many species of trypanosomes transmitted by tsetse-flies, a peculiar 

 mode of development occurs, as already stated, in the proboscis, termed 

 by Roubaud, who discovered it, a culture d'attente. The trypanosomes 

 taken up from the vertebrate change very rapidly into trypanomonad 

 ("leptomonad," Roubaud) forms, with the kinetonucleus far forward, and 

 attach themselves by the tip of the flagellum to the wall of the proboscis tube. 

 In this situation they multiply in the salivary fluid by binary fission, until 

 great numbers are present. In some cases this culture in the proboscis appears 

 to be the sole form of developmental cycle in the fly, as, for example in 

 T. cazalboui (Roubaud, 506, Bouffard), T, vivax (Bruce, 411, iii.) ; this type 

 is termed by Roubaud evolution par fixation directe. In other species 

 (T. dimorphon, T. pecaudi) the parasite multiplies first in the digestive tract 

 of the fly, and then spreads forward into the proboscis -evolution par fixation 

 indirecte of Boubaud ; in this case, however, the possibility does not seem 

 to be excluded that the forms seen in the digestive tract may have belonged 

 to the developmental cycle of a distinct trypanosome. Development of this 

 kind has only been observed in tsetse-flies. 



According to Bouffard, T. cazalboui can be transmitted mechanically by 

 Stomoxys, but goes through its developmental cycle only in the proboscis of 

 Olossina palpalis ; Stomovys may therefore cause epidemics of the disease 

 (" souma "), but endemic areas are always in regions where O. palpalis 

 occurs. The tsetse-fly is not infective until six days after first feeding on an 

 infected animal, and it then remains infective permanently, or at least for the 

 greater part of its existence. Hence the proboscis- cycle is a rapid develop- 

 ment, comparable, as regards the time it requires, to that of T. lewisi in the 

 flea rather than to that of other trypanosomes in the digestive tract of the 

 tsetse. 



Finally,* mention must be made of the cysts of T. grayi, described by 

 Minchin (476), occurring in the hind-gut of Olossina palpalis. The cysts result 

 from the encystment of a crithidial form, and are very similar to the cysts of 

 Herpetomonas, described by Prowazek (Pig. 124), from the hind-gut of the 

 house-fly ; their mode of formation indicates that they are destined to pass 

 out of the rectum to the exterior with the faeces, and Minchin has suggested 

 that a contaminative method of spreading the infection may occur in addition 

 to the usual inoculative method. The possibility must be reckoned with, 

 however, that the cysts in question may be part of the cycle of a distinct 

 flagellate parasite, perhaps peculiar to the fly alone, and may not belong at all 

 to the life- cycle of T. grayi, which has now been shown to be the developmental 

 form of the trypanosome of the crocodile (cf. Cystotrypanosama, Roubaud, 

 557 - 5). According to Kleine and Taute, trypanosomes, not encysted, may 

 be found in the faeces of infected tsetses. 



Apart from the somewhat aberrant development of the members 

 of the 6rwctt-group, which require further elucidation, the cycle 

 of a trypanosome in the invertebrate host appears to consist typically 

 of three principal phases : (1) An initial multiplicative phase, which 

 may be trypaniform, as in T. lewisi, or Leishmania - like, as in 

 T. cruzi, or may take the form of unequal division of large trypani- 

 form individuals to produce either small crithidial forms directly, 

 as in fish-trypanosomes in the leech Hemiclepsis, or rounded 

 Leishmania-torma which later become crithidial, as in T. raice 

 and T. vittacet ; to this initial phase succeeds (2) a crithidial phase, 

 which may pass farther down the alimentary canal, and which in any 

 case multiplies by fission and constitutes the principal stock of the 



* The development described by Schaudinn (132) for T. noctuee is dealt with in 

 a subsequent chapter (p. 390). 



