140 University of California Publications in Zoology [VOL. 19 



pathogenic to its vertebrate host, the rat. The invertebrate host of 

 this haemoflagellate is normally the rat-flea, which transmits the 

 flagellate from rat to rat. Both the vertebrate and the invertebrate 

 host of T. lewisi are widespread, abundant, easily procurable, and 

 adaptable to laboratory conditions. To make certain that none of the 

 stages of the life history of the so-called natural flagellates should 

 be confused with stages of the life history of T. lewisi a stock of un- 

 infected fleas was procured for the breeding cages. The rat-fleas have 

 frequently been found to be infected with Leptomonas pattoni. 



At the time of the publication of the Minchin and Thomson paper 

 I was working on the morphology and life history of Crithidia eury- 

 ophthalmi (McCulloch, 1917), a form which occurs in the alimentary 

 tract of Euryophthalmus convivus. This material was of great in- 

 terest; the intracellular process of multiple fission was found, and 

 the initial infective spores were relatively abundant. Previously 

 some time had been spent in studying the morphology and life history 

 of Crithidia leptocoridis (McCulloch, 1915), which infects the digestive 

 tract of the box elder bug, Leptocoris trivittatus. It had also been 

 pointed out in a superficial way that the individuals of various forms, 

 shapes, and structures found in a typical infection of this crithidia, 

 C. leptocoridis, were apparently analogous to many of the correspond- 

 ing figures of Schizotrypanum cruzi (Chagas, 1909) in the invertebrate 

 host. As the investigation of C. euryophthalmi and C. leptocoridis 

 proceeded it became increasingly easy to link the life history of 

 Critkidia with the life history of Trypanosoma in the invertebrate 

 host. This was especially true of the life history of T. lewisi. In 

 order to demonstrate clearly the essential facts of the life cycle of one 

 of these important flagellates a brief outline of the life history of 

 T. lewisi will be given, based upon the work of Minchin and Thomson. 



The developmental cycle of T. lewisi in the flea is divided into two 

 phases, characteristic of the parts of the digestive tract in which the 

 trypanosomes are found, viz., the stomach and the rectal phase. The 

 cycle in the rat-flea requires a minimum of five days for its complete 

 course. The trypanosomes enter the stomach of the flea with the 

 blood of an infected rat. They show the characteristic structure of a 

 trypanosome, the " kinetonucleus, " or parabasal body, being posterior 

 to the nucleus and the undulating membrane well developed. The 

 change in the medium is probably responsible for the physiological 

 changes which result in the bodies becoming more cylindrical. The 

 trypanosomes then penetrate the epithelial cells of the stomach and 



