342 FAMILY: TRYPANOSOMID^ 



forms — which, owing to deposit round them on the film, have taken 

 on the appearance of cysts — it has to be assumed that the flagellate of 

 the tsetse fly not only passes from fly to crocodile, but also from fly to fly 

 by means of cysts; or that the crocodile becomes infected by ingesting the 

 cysts passed in the faeces of the fly. The discovery by Lloyd (1924) of 

 a typical Leptomonas in the labial cavity of the proboscis and the mid-gut 

 of Glossina morsitans still further complicates the question of the nature 

 of the so-called cysts of these flagellates. 



In the process of encystment of H. muscarum, the flagellate first 

 retracts itself to an ovoid body around which the cyst forms. The retrac- 

 tion, however, may take place in three different ways, as will be described 

 below (Fig. 171). That the cysts of insect flagellates are actually re- 

 sistant bodies has been proved by the writer (1912c) in the case of the 

 leptomonas of the flea, Pulex irritans (p. 351). It must be remarked, 

 however, that it is exceedingly difficult in most cases to form a definite 

 opinion as to whether a minute leishmania form, as seen in the hind-gut 

 of an insect, is actually encysted or not, and the mere fact that in 

 dried films stained by Romanowsky stain a red line surrounds them can 

 hardly be regarded as evidence of the presence of a cyst wall. The thick 

 envelopes which have been figured as cyst walls by numerous observers 

 are probably artefacts due to the staining of granular material which has 

 become heaped round the leishmania forms in the process of drying the 

 films. In other cases, the structures described as cysts have probably 

 been yeasts, or even spores of microsporidia. That a membrane actually 

 exists seems to be proved by the resistance of these forms to drying, but it 

 must be admitted that the detection of a cyst wall is a far more difficult 

 matter than many have supposed. Hoare (1923), in his work on the 

 development of the trypanosome of the sheep in the ked, draws attention 

 to the possibility of mistaking yeasts or other artefacts for cysts, and has 

 shown that the supposed cysts of the flagellate of the ked, which w^as at 

 one time thought to be a parasite peculiar to this insect, are merely 

 rounded forms, with deposits of stained granular material round them, 

 or yeasts. The cysts of H. grayi may be capable of a similar interpreta- 

 tion (Fig. 173). The changes in nuclear structure which occur during 

 the alleged encystation of this flagellate may be merely evidence of 

 degeneration. 



GENERAL FEATURES OF THE LIFE-HISTORY. 



In the description of genera given above it has been explained 

 that the flagellates belonging to the genera Herpetomonas, Crithidia, and 

 Leptomonas have only an invertebrate host, while those belonging to the 

 genera Trypanosoma and Leishmania have both vertebrate and inverte- 



