654 TRYPANOSOMIASIS 



the male form and the latter the female, and intermediate or 

 indifferent types are also seen. Whether any significance is to 

 be attached to the occurrence of these different types is at 

 present unknown, but it is probable that some of them have 

 more vegetative activity than others, and the prevalence of these 

 is related to the infectivity of the blood when transferred to a 

 new host. Further, in especially chronic infections the number 

 of organisms present in the peripheral blood varies, and thus the 

 potentiality of infection by means of an invertebrate carrier also 

 varies. When the organisms are absent from the blood they 

 may still be found in the solid organs and in the bone marrow, 

 and in such situations may go through a resting phase of 

 development. In Tr. cruzi such a stage has been demonstrated 

 in endothelial cells, and a similar condition has been observed in 

 Tr. brucei in the gerbil. 



The outstanding fact in the biology of the pathogenic 

 trypanosomes is that infection from vertebrate to vertebrate 

 takes place by the parasite being transferred by the agency of 

 biting or blood-sucking insects, or by leeches. The mere 

 mechanical transference by such invertebrates is possible, and 

 in certain cases a multiplication of the organism in the biting 

 apparatus of the invertebrate occurs. Such a mechanical or 

 semi-mechanical transference plays, however, a subsidiary part 

 in ordinary infections, for in many cases a considerable period 

 may elapse between, e.g., an insect taking up infective bloo'd 

 and becoming itself infective for new hosts. Here the parasite 

 undoubtedly goes through a cycle of development within the 

 invertebrate, the details of which are in many instances as yet 

 undetermined. In the blood taken up, the trypanosomes are 

 seen to undergo modifications in form. They may show simple 

 division by which the resulting individuals become smaller, — the 

 relation of kinetonucleus and trophonucleus may be altered, — 

 and the undulating membrane and flagellum become rudi- 

 mentary (crithidial forms). In other cases, organisms resembling 

 Leishmanise result. The stage in the cycle at which the organ- 

 ism again becomes infective for the vertebrate host differs in 

 different instances. 



There are probably great differences in the cycles of trypano- 

 somes within the vertebrate and invertebrate hosts, and contro- 

 versy has turned round the question of whether a sexual con- 

 jugation occurs. This has been described in connection with the 

 so-called male and female adult forms of the trypanosome 

 already described, and also in connection with crithidial forms. 

 While the analogy of what happens in the malarial parasite 



