170 University of California Publications in Zoology [VOL. 19 



tion of a zygote, as does the somatella of Plasmodium in the wall of 

 the digestive tract of a mosquito. There is no evidence that the zygote 

 precedes the somatella in the Polymastigina. In the life cycle of 

 Trypanosoma lewisi the process of intracellular multiple fission is 

 apparently obligatory. The trypanosomes, or haemoflagellates, pene- 

 trate epithelial cells of the "crop," undergo multiple fission, and 

 crithidiomorphic merozoites are produced. This phase brings about the 

 early stages of transition from a trypanosome to a crithidia. A similar 

 need for such a phase is not present in the life cycle of Crithidia 

 euryophthalmi. At present our knowledge of intracellular multiple 

 fission in the life cycle of C. euryophthalmi is too meager to permit of 

 extended correlation between the life cycle of this more primitive 

 flagellate with that of the more highly developed haemoflagellate T. 

 leivisi. It is conceivable that in the evolution of trypanosomes from 

 the crithidial-like flagellates the intracellular multiple fission was 

 carried over and became more specialized and more important in the 

 life cycle of the haemoflagellate. 



RECTAL PHASE 



The stomach phase of Crithidia euryophthalmi, beginning with the 

 initial infective spores and ending with the great swarm of parasites 

 resulting from binary, extracellular, and intracellular multiple fission 

 in the "crop," is followed by the established rectal phase of the life 

 cycle in the pyloric expansion. 



Owing to the structure of the digestive tract of Euryophthalmus 

 convivus, with its three divisions separated only by a narrow con- 

 striction, which allows a possible intermingling under normal condi- 

 tions of the crithidias in the "crop" with those of the mid-stomach 

 and pyloric expansion, it is extremely difficult to say where the stomach 

 phase ends and the rectal phase begins. In Trypanosoma lewisi the 

 transition between the stomach and rectal phase, as has already been 

 pointed out, is marked by definite structural changes. The trypano- 

 somes, brought into the stomach of the flea with blood from a rat, enter 

 epithelial cells and undergo multiple fission, producing merozoites of 

 a crithidiomorphic type. The merozoites thus produced may do one 

 of two things, either enter other epithelial cells of the stomach or 

 collect at the pyloric opening and be carried down the intestine to the 

 rectum with food. As they pass through the intestine the structural 



