306 THE PROTOZOA 



divide up into trypanosomes of the ordinary type. Those who consider that 

 syngamy can only occur in the invertebrate host will doubtless regard the 

 process described by Ottolenghi as phenomena of agglomeration and de- 

 generation. In the present state of our knowledge, however, it is best to keep 

 an open mind on this question, and to await further investigations. 



In T. gambiense, Moore and Breinl (484) have described a process of fusion 

 between the kinetonucleus and trophonucleus in the formation of the " latent 

 bodies," and have interpreted this as a sexual process, a suggestion hardly 

 to be taken seriously. A similar process alleged to occur in the multiplication - 

 forms of T. lewisi has been interpreted by Schilling as the inevitable autogamy. 



All that can be said at present, with regard to sexual processes in trypano- 

 somes, is that, on the analogy of other Protozoa generally, syngamy may be 

 expected to occur in some part of the life- cycle. It remains, however, for 

 further research to establish definitely the conditions under which syngamy 

 takes place, and the nature of the process in these organisms ; nor can it be 

 considered as sound reasoning, in the absence of concrete observations, to at- 

 tempt to limit the possible occurrence of syngamy, or to infer the exact form 

 it takes, either by analogies more or less far-fetched with one or another 

 group of Protozoa, or by the mere existence of form-differentiation, and still 

 less by the arbitrary interpretation of certain forms as zygotes or ookinetes. 



A very variable feature in the development of trypanosomes is the sus- 

 ceptibility of the invertebrate host. In the case of T. lewisi, only about 

 20 per cent., approximately, of the fleas fed experimentally on infected rats 

 become infective in their turn, and in the case of tsetse-flies and pathogenic 

 trypanosomes the percentage is much smaller. There are also grounds for 

 suspecting that a certain condition or phase of the trypanosome in the blood 

 of the vertebrate is sometimes necessary for establishing the developmental 

 cycle in the invertebrate ; compare the observations and conclusions of Chagas 

 with regard to Schizotrypanum cruzi, mentioned above (p. 296). In Trypano- 

 soma noctuce the summer form which appears in the blood is of a type distinct 

 from the winter forms found in the bone-marrow (Minchin and Woodcock, 42). 

 On the other hand, in the case of the trypanosomes of fresh- water fishes, Robert- 

 son (503) found that every leech became infected that was fed on an infected 

 fish ; so that the simplest method of determining whether a fish was infected 

 was to feed a newly-hatched Hemidepsis on it. 



A question often discussed is whether trypanosomes in any part of their 

 development may pass through " ultramicroscopic " stages. Schaudinn (132) 

 expressed the opinion that some stages of trypanosomes investigated by him 

 were small enough to pass through bacterial filters ; though he did not put 

 this suggestion to an experimental test, it is often quoted as a proved fact.* 

 Moore and Breinl (484) also asserted, without experimental data, that infected 

 blood remained infective after filtration. On the other hand, attempts by 

 Bruce and Bateman to obtain experimental verification of these statements 

 gave negative results (compare also Report XI., p. 122, of the Sleeping Sickness 

 Commission). 



Recently it has been asserted by Fry that T. brucii can throw off granules 

 which, when liberated, possess a certain motility of their own in the blood ; 

 this process is regarded as " essentially of a vital and not a degenerative 

 nature." That a trypanosome or any other living cell might excrete grains 

 which when set free could exhibit movements due to molecular or other 

 causes is highly probable ; but that such grains represent a stage in the life- 

 history of a trypanosome is far from being so ; nor can analogy with spiro- 

 chaetes be considered as a legitimate argument in favour of any such con- 

 clusion. 



There remains for consideration the development which trypanosomes 

 undergo in artificial cultures, in which they exhibit a series of forms quite 

 different from those seen in the blood of the vertebrate, and so far resembling 

 the cycle in the invertebrate host in that the predominant phase is a crithidial 



* It is doubtful whether the forms of which Schaudinn made this statement were 

 really trypanosomes or spirochsetes. 



