BIOLOGY OF THE TRYPANOSOMES 653 



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 certain cases (Tr. cruzi) such a stage has been 

 demonstrated in endothelial cells. In this the nucleus becomes 

 condensed and divides to form merozoite-like bodies from which 

 an infection of red blood corpuscles occurs. In these the 

 trypanosomal form is reproduced. A similar stage 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 blood 

 and becoming itself infective for new hosts. Here the parasite 

 undoubtedly goes through a cycle of development within the 

 invertebrate, the details of which vary in different cases and at 

 present are in many instances as yet undetermined. In the 

 blood taken up, the trypanosomes are seen to undergo modifica- 

 tions in form. They may show simple division by which the 

 resulting individuals become smaller, the relation of kineto- 

 nucleus and trophonucleus may be altered, and the undulating 

 membrane and flagellum become rudimentary (crithidial forms). 

 In other cases, organisms resembling Leishmaniae result. The 

 stage in the cycle at which the organism 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 conju- 

 gation occurs. This has been described in connection with the 



