1919] McCulloch: Life Cycle of Crithidia and Trypanosoma 141 



undergo the process of multiple fission, a process which characterizes 

 the stomach phase. The initial steps in this process after the trypano- 

 some has entered an epithelial cell of the stomach, is the formation 

 of a "tailed" sphere. This is a spherical structure with the flagellum 

 and some cytoplasm protruding at one point. A little later the 

 the ' ' tailed ' ' sphere becomes ' ' tailless, ' ' and shows internally the form- 

 ation of a variable number of daughter individuals. In the forma- 

 tion of these daughter individuals, the nucleus, parabasal body, and 

 blepharoplast each divide an equal number of times, forming mero- 

 zoites of a crithidiomorphic type, i.e., a long, free type of trypano- 

 some whose external movements are crithidial but whose structure still 

 shows the parabasal body posterior to the nucleus. The crithidio- 

 morphic flagellates may do one of two things: enter other epithelial 

 cells and undergo the process of multiple fission, or collect at the 

 pyloric opening of the stomach, to be carried down through the intes- 

 tine into the rectum with the food. The intestine ordinarily serves as 

 a passageway for the parasites from the stomach to the rectum, but 

 under certain conditions rectal forms may migrate forward and attach 

 themselves to the wall of the intestine in the postpyloric region. 



The rectal phase is established by the entrance of the crithidio- 

 morphic forms into this region of the digestive tract, and this region 

 becomes the permanent source of infection throughout the life of the 

 flea. During the migration from the stomach to the rectum structural 

 changes take place in the crithidiomorphic trypanosomes. The poste- 

 rior end of the body becomes club-shaped, and this shifting of the 

 cytoplasm assists in the forward movement of the parabasal body to 

 a position anterior to the nucleus. Following this condition, binary 

 fission brings about the production of the smaller, minute crithidias 

 which characterize this phase of the life history. The established rectal 

 phase is described as consisting of: (1) the attached or haptomonad 

 form, which is the multiplicative stage of the rectal development; 

 (2) the free or nectomonad form; and (3) the final trypanosome form 

 which is instrumental in infecting another host. 



The above outline of the life history deals only with the develop- 

 mental series; but in both the stomach and rectum there are present 

 individuals which show degeneration and belong consequently to the 

 degenerative series. I have found that the life history of Crithidia 

 euryophthalmi can be correlated advantageously with the life his- 

 tory of T. lewisi in the invertebrate host, the flea. The life history 

 of C. euryophthalmi is not the life history of a haemoflagellate but 



