GENUS: TRYPANOSOMA 455 



ovinus), and came to the conclusion that they were merely stages of one 

 organism {T. melo-phagium), a fact which has been conclusively demon- 

 strated by Hoare (1923). 



The course of development of trypanosomes in cultures is of considerable 

 interest, for, undoubtedly, it is an imitation of the development which 

 takes place normally in the invertebrate host. Most observers have 

 noted that the trypanosomes introduced into culture media commenced 

 multiplying and became transformed into flagellates of the crithidia type. 

 Thomson, J.D. (1908), first noted in the case of cultures of the trypanosome 

 of gold fish that the first division process of the trypanosomes resulted 

 in the formation of crithidia forms (Fig. 247). Active multiplication of 

 these takes place for some time, but eventually trypanosome forms again 

 appear. There seems little reason to doubt that these represent the meta- 

 cyclic trypanosomes which appear at the end of the development in the 

 leech. Delanoe (1911) noted that in old cultures of T. lewisi small try- 

 panosomes appear, and it is evident from his figures that these correspond 

 with the metacyclic trypanosomes which are developed in the rectum of 

 fleas, and which are the actual infective forms. Hoare (1922, 1923), 

 working with cultures of T. melophagium, both from the blood of sheep 

 and from the intestine of the ked, has noted the same fact. Small trypano- 

 somes appear in the cultures after the crithidial phase, and these are 

 identical with the metacyclic trypanosomes which are developed naturally 

 in the hind-gut of the ked. It seems clear, therefore, that the course of 

 development of any trypanosome in culture is a parallel of the natural 

 development in the invertebrate. Noller (1920c), working with cultures 

 of T. loxicB and T. syrnii of birds, T. theileri of cattle, and T. melophagiimi 

 of sheep on blood-agar plates, states that at low temperatures the flagellates 

 remain in the crithidia form, but that elevation of the temperature to 

 37° C. causes their transformation into trypanosomes. It seems that the 

 only possible biological explanation of this phenomenon is that the 

 heightened temperature causes the flagellates to revert to the warm- 

 blooded vertebrate phase, which is a step in advance of the metacyclic 

 trypanosomes which appear in the cultures in liquid media at low tem- 

 peratures. It would seem reasonable to suppose that the transforma- 

 tion noted by Noller as occurring on agar plates after an elevation of 

 temperature is not comparable with the appearance of trypanosomes in 

 liquid media kept at a uniformly low temperature, but rather with the 

 changes undergone by the metacyclic trypanosomes after they gain 

 entrance to a vertebrate. 



7. INSECT VECTOR. — Finally, the capacity to develop in invertebrate 

 hosts may be employed as a means of difEerentiating trypanosomes. 

 T. gambiense is capable of infecting Glossina palpalis, and only rarely 



