588 SUMMARY OF CURRENT RESEARCHES RELATING TO 



The rectal phase consists chiefly of small, often minute individuals, 

 which are crithidial in structure, and are attached by the tip of the 

 flagellum to the wall of the rectum, where they keep up a continual 

 multiplication by binary fission. This phase probably endures as long 

 as the flea lives, and thus it may remain infective without any renewal of 

 the infection. From this crithidial phase arise, by modification of indi- 

 vidual forms, the small trypanosome-forms by which infection of the rat 

 is brought about, and which are the final forms of the developmental 

 cycle in the flea. It is interesting, however, that only a small propor- 

 tion of fleas fed on infected rats become infective. Trypanosomes may 

 be found in the gut of all fleas examined soon after feeding, but in 

 about 75 p.c. the parasites die out without completing the developmental 

 cycle. It is therefore necessary to distinguish a series of degenerative 

 forms from the developmental series. To this section of the paper are 

 appended (1) a history of previous investigations on the development of 

 Trypanosoma lewisi, and (2) a note on the possibility of the occurrence 

 of sexual phenomena in T. lewisi. 



The final section of the paper describes the experimental study of 

 the problems of transmission and development, with the following 

 results. The rat trypanosome (T. lewisi) is transmitted from rat to 

 rat by the rat-flea Ceratophyllus fasciatus. The transmission takes 

 place by the cyclical method ; transmission by the direct method has 

 not been proved to occur. The trypanosomes make their appearance in 

 the blood of the rat five to seven days after infection ; the multiplica- 

 tion of the trypanosomes in the blood of the rat comes to an end eleven 

 to thirteen days after infection. The cycle of development in the flea 

 requires a minimum of five days for its completion Transmission is 

 never effected until the developmental cycle is completed, that is, until 

 at least five days after the first exposure of the fleas to infection. The 

 developmental forms of trypanosome in the flea are not infective when 

 inoculated into the rat during a period extending from a short time 

 (half-an-hour ?) after being taken up by the flea until after the develop- 

 ment is complete. The rat may become infected by eating infected 

 fleas, but not until the developmental cycle of the parasite within the flea 

 is complete. Infection of the rat is effected contaminatively by way of 

 the rat's mouth, by the rat licking from its fur or skin the moist fceces 

 of fleas containing the final propagative form of the cycle. There is no 

 evidence that the flea can infect the rat by inoculating trypanosomes 

 into it through the proboscis. There is no evidence that hereditary 

 transmission of trypanosomes from flea to flea ever takes place. The 

 trypanosomes in the blood of the rat can render fleas infective soon after 

 tbey make their appearance in the blood, before their multiplication 

 period is over. Starvation of the flea during the incubation period does 

 not inhibit, or even necessarily retard the developmental cycle of the 

 trypanosome in the flea, but varying food conditions do have some 

 influence on the location and incidence of the Haptomonad phase in the 

 flea's gut. 



