THE PATHOGENIC TRYPANOSOMES 547 



Multiplication goes on readily on such a medium, the 

 organisms dividing longitudinally as in the tissues. Sometimes 

 very small forms result, and often these are found in rosettes 

 which are formed by a number of individuals arranging them- 

 selves in a circle with the flagella directed towards the centre 

 of the agglomeration. By repeated sub-cultures several of the 

 trypanosomata named have been kept alive for more than a year, 

 and when re-introduced into appropriate hosts have been found 

 not to have lost their infective properties. 



Within recent years considerable attention has been directed 

 to the question of whether in the trypanosomes a sexual 

 cycle occurs. It cannot be said that the existence of such has 

 been definitely established, and we shall merely give a short 

 account of certain views which have been advanced. The 

 starting point lies in the slight differences in form which have 

 been observed in the organisms in the body fluids of the 

 vertebrate hosts. Such differences have been described in 

 Tr. Lewisi and Tr. Brucei by Prowazek, and in Tr. ugandense 

 by Minchin, and have been made the basis of a classification into 

 three types which are looked on as representing male, female, and 

 indifferent individuals. The male type is rather slender both in 

 body and in nucleus, the free part of the flagellum is longer than 

 the body and the protoplasm is free from granules ; the female 

 is broader, its nucleus is larger and rounder, the undulating 

 membrane narrower, the free part of the flagellum is shorter 

 than the body, and the protoplasm contains many chromatin 

 granules, which are looked upon as reserve food material. The 

 indifferent individuals present intermediate characters. All 

 multiply by fission as described, and the indifferent individuals 

 can on occasion become differentiated into male or female 

 forms. The females are the most hardy, and next come the 

 indifferent individuals. If all but the females die out, these 

 can undergo .parthenogenesis, and representatives of all three 

 types can be again reproduced. The sexual cycle is represented 

 as occurring in the invertebrate host. In Tr. Lewisi, according 

 to Prowazek, this is found in the rat louse, hcematopinus 

 spinulosus. When this insect sucks the blood of an infected 

 rat, copulation occurs by the male trypanosome entering the 

 female near the micronucleus and the various parts of the two 

 individuals becoming fused. A non-flagellated ookinete results, 

 which, passing through a spindle-shaped gregarine-like stage, 

 can develop into a trypanosome in the stomach of the louse. 

 A resting-stage in an immature trypanosome -like form is 

 described as occurring between or attached to the intestinal 



