THE PATHOGENIC TRYPANOSOMES 613 



alive for more than a year, and when re-introduced into appro- 

 priate hosts have been found not to have lost their infective 

 properties. 



The main fact in the biology of those trypanosomata with 

 which the pathologist is concerned, is that in the higher animals 

 infection takes place by the parasite being transferred from one 

 host to another by the agency of biting or blood-sucking insects, 

 or by other similar agencies such as leeches. It may be said 

 that the mere mechanical transference of the parasite by, say, 

 a blood-flocking insect, while it may sometimes occur, probably 

 plays a subsidiary part in infection. Several instances will be 

 given in which it is known that an insect does not become 

 actively infective until some days have elapsed after it has 

 sucked the blood of an infected animal. The analogy of the 

 malarial organisms suggests the occurrence of a sexual con- 

 jugation within the insect, but definite proof of this is still 

 wanting, and, as Minchin points out, while we must admit the 

 existence of a cyclic development, it by no means follows that 

 this includes a definitely sexual stage, although many are of 

 opinion that such a stage does take place. 



The starting-point of the sexual theory 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. 

 Leuisi 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, according to the supporters of the sexual theory, 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 in- 

 vertebrate host. In Tr. Lewisi. according to Prowazek, this is found 

 in the rat louse, hwmaiopinus .yrinulosus. "When this insect sucks the 

 blood of an infected rat, copulation occurs by the male trypanosome 

 entering tin- female near the micronucleus and the various parts of the 

 two individual! becoming fused. A n on -flagellated obkinete results, 

 which, passing through a spindle-shaped grcgarinc-like stage (crUhidium), 

 can develop into a trypauosome in the stomach of the louse. A resting- 

 stage in an immature trypanosotne-like form is described as occurring 

 in or on the intestinal" epithelium, and the parasite is supposed to 

 reach the body cavity, and ultimately the pharynx of the insect, and 



