THE H^MOFLAGELLATES AND ALLIED FORMS 303 



The three principal phases in the development of T. cruzi in the bug may be 

 compared, without difficulty, with those of T. lewisi in the flea, though 

 differing in minor details ; in both cases an early multiplicative phase in the 

 stomach is followed by a crithidial phase, also multiplicative and constituting 

 the principal stock of the parasite, in the hinder part of the digestive tract ; 

 to this succeeds a propagative trypaniform phase, which in the case of T. leurisi 

 passes forwards to the stomach, but which in the case of T. cruzi appears to 

 pass through the wall of the alimentary canal into the body- cavity, and so 

 into the salivary glands. Other developmental forms have been described 

 by Chagas, but their relation to the cycle of the parasite, if indeed they really 

 belong to it, is not clear. 



The developmental cycle of the trypanosomes of fresh- water fishes in the 

 leech Hemiclepsis marginata (Robertson, 503) begins also by active multi- 

 plication in the crop about six to nine hours after the flagellates have been 

 ingested. The trypanosomes divide by repeated binary fission of unequal 

 type, budding off small individuals which are crithidial in type and multiply 

 in their turn. In a few days the crop is populated by a swarm of trypano- 

 monad forms of various sizes, multiplying actively. Towards the end of the 

 digestion, the propagative phase begins to appear in the form of long, slender 

 trypaniform individuals which arise directly from the crithidial forms, and 

 pass forwards in great numbers from the crop into the proboscis-sheath, 

 whence they are inoculated by the leech into a fresh host. A certain number 

 of the crithidial forms remain behind in the crop, however, where during 

 hunger- periods they may pass into a resting Leishmania-iorm ; when the 

 crop is again filled with fresh blood, these forms begin to multiply again, 

 repopulating the crop with crithidial forms, from which a fresh batch of 

 trypaniform propagative individuals arise towards the end of digestion 

 again. 



In the development of T. raice in the leech Pontobdella muricata (Robertson, 

 500, 502), the ingested trypanosomes multiply in the crop in a similar manner 

 by unequal binary fission, budding off small individuals which, however, are 

 rounded and leishmanial in type, and which pass down from the crop into the 

 intestine, where they develop a flagellum, become crithidial in type, and 

 multiply actively. During hunger- periods they become leishmanial, resting 

 forms which persist when all other forms have succumbed and died out, becom- 

 ing crithidial again when the supply of food is renewed. From the crithidial 

 forms arise the long, slender trypaniform individuals of the propagative phase, 

 which pass forward into the proboscis to be inoculated into the fish. The 

 development of T. vittatce, from the blood of the Ceylon tortoise, Emyda vittata, 

 in the leech Glossiphonia sp., is of a similar type, but takes place almost entirely 

 in the crop (Robertson, 501). 



The development of T. gambiense in the tsetse-fly, Glossina palpalis, so far 

 as it has been described by Kleine (457), Kleine and Taute (459), and Bruce 

 and his collaborators (415), presents some peculiar features not quite intelli- 

 gible at present. The whole development takes a long time, about eighteen 

 to twenty- five days or more, a fact which, together with the low percentage 

 of flies which become infected, accounts for the existence of a developmental 

 cycle having been missed by so many investigators, until it was first dis- 

 covered by Kleine. From five to seven days after the infection of the fly the 

 trypanosomes disappear or become scarce in its digestive tract, indicating, 

 possibly, an intracellular stage yet to be discovered. Later, in a small 

 percentage of the flies, the trypanosomes reappear in the digestive tract in 

 enormous numbers. The flagellates at this stage vary greatly in size, form, 

 and appearance, but crithidial forms are stated to be very rare, a feature 

 in which the development contrasts with the usual type seen in other trypano- 

 somes. Finally an invasion of the salivary glands takes place, though in what 

 way it is brought about is not clear ; short, stumpy trypaniform individuals 

 are found in the glands, which represent the ripe, propagative phase destined 

 to be inoculated into the vertebrate host. These ripe forms first make their 

 appearance, according to Kleine, in the intestine. 



