THE H^EMOFLAGELLATES AND ALLIED FORMS 301 



brane running the length of the body is established. The trypani- 

 form individuals thus formed are of small size and broad, stumpy 

 form ; they represent the propagative phase which passes from the 

 flea back into the rat. From the rectum they pass forwards into 

 the stomach, and from the stomach they appear to be regurgitated 

 into the rat's blood when the flea feeds. 



Experiments show that the flea becomes infective to the rat in 

 about six days after it first took up the trypanosomes from an 

 infected rat. The intracellular phase is at the height of its develop- 

 ment about twenty-four hours after the flea takes up the trypano- 

 somes ; the rectal phase begins to be established towards the end of 



FIG. 131. Trypanosoma lewisi: developmental phases from the rectum of the rat- 

 flea. A, Early rectal form ; B, crithidial form attached to wall of rectum ; 

 G, D, division of crithidial form ; E, clump of crithidial forms detached from 

 wall of rectum, hanging together by their flagella, one of them beginning to 

 divide ; F, G, H, crithidial forms without free flagella ; /, rounded form 

 without flagellum ; J, K, L, M, series of forms transitional from the crithidial 

 to the final trypaniform type ; N, the last stage in the flea. Magnified 2,000. 



the first or beginning of the second day ; and the stumpy, trypani- 

 form, propagative phase is developed in the rectum towards the end 

 of the fifth day. 



The account of the development of T. lewisi in the flea given in the fore- 

 going paragraphs is based upon investigations, some of them aSKy&t unpub- 

 lished, carried on in conjunction with Dr. J. D. Thomson by the author 

 (480-482). Some of the phases of the parasite have also been described by 

 Swellengrebel and Strickland (517). A number of investigators namely, 

 Prowazek (497), Breinl and Hindle, Baldrey (396), Rodenwaldt, and others- 

 have studied the development of this trypanosome in the rat-louse (Hcemato- 

 pinus spinulosus). Experiments have shown that this insect is also capable 

 of transmitting the trypanosome from rat to rat, but only, to judge from the 



