292 THE PROTOZOA 



their transmission have not yet been worked out in a satisfactory or 

 conclusive manner. 



It must be considered for the present an open question whether true try- 

 panosomes occur as parasites of an invertebrate host exclusively ; the answer 

 to the question will depend on the significance given to the expression " true 

 trypanosome." It is now practically certain that many leptomonads have a 

 trypaniform phase in their development (see p. 314, infra), so-called " Icpto- 

 trypanosomes." In Drosophila confusa, a non- biting, muscid fly, Chatton 

 and Alilaire (compare also Chatton and Leger) found in the Malpighian tubules 

 a trypaniform type of flagellate which they consider as a " eutrypanosome," 

 as a species of Trypanosoma distinct from the Leptomonas occurring in the 

 gut of the same fly (Fig. 137). Wenyon (84) also found similar forms in the 

 Malpighian tubules of house-flies in Bagdad, and considered that they might 

 belong to the cycle of the Leptomonas (Herpetomonas) in the same host. In 

 both cases the phase in the Malpighian tubules is a little stumpy trypanosome- 

 like form, very similar in its characters to T. nanum. The fact that these 

 " eutrypanosomes " are so far known only to occur in flies which are infected 

 also by a species of Leptomonas indicates that, like the " leptotrypanosomes," 

 they are merely a phase in the cycle of the Leptomonas. 



From the foregoing it is seen that the complete life-cycle of a 

 trypanosome is an alternation of generations corresponding to an 

 alternation of hosts. One part of the cycle is passed in the blood of 

 a vertebrate, in which the predominant form is the trypanosome- 

 type of flagellate ; the second part is passed in the digestive tract 

 of an invertebrate, and here the predominant form is the crithidial 

 or trypanomonad type. We may consider the life-history, therefore, 

 under these two principal phases : 



1. As a type of the life-cycle in the vertebrate host, that of the 

 common rat-trypanosome may be taken. After infection, natural 

 or artificial, of the rat, the trypanosomes make their appearance 

 in the blood about the fifth, sixth, or seventh day. What the para- 

 sites have been doing during this time, the so-called " incubation- 

 period " in the rat, cannot as yet be stated definitely ; it may be 

 that the relatively few trypanosomes inoculated by the flea or 

 syringe have merely been multiplying steadily, in the manner 

 presently to be described, until they become sufficiently numerous 

 in the blood to be detected by microscopic examination ; there may, 

 on the other hand, be phases of the parasite as yet unknown during 

 this period, and, according to recent statements (Carini, 422), a 

 process of schizogony takes place in the lung similar to that dis- 

 covered by Chagas in Schizotrypanum cruzi (see below). 



When the trypanosomes first appear in the blood, their most 

 striking peculiarity is the extraordinary diversity in type which they 

 exihibit. Besides " ordinary " individuals of the normal dimensions 

 of the " adult " form, there are others smaller or larger, the extremes 

 of size being relatively huge in one direction, very minute in the 

 other. These differences of size are due to the fact that the try- 

 panosomes are multiplying actively, the large forms being those 



