CYST FOKMATION 341 



ENCYSTATION. 



In those Trypanosomidse which are confined to insect hosts, infection 

 is spread from one to another by encysted forms passed in the fseces. In 

 the trypanosomes, however, cysts do not occur, as infection takes place 

 either by the inoculation of free forms through the proboscis, or by the 

 ingestion by the vertebrate of free forms passed in the faeces of the insect. 

 The cyst, in the case of the purely insect flagellates, is formed in the hind- 

 gut or rectum around small leishmania forms which are produced there 

 by the gradual shortening of the elongated forms (Fig. 150, 13-16, 24-26, 

 46-50). They are described as differing from the unencysted leishmania 

 forms in that they have a much more definite and deeply stained outline. 

 In some cases the cyst wall is depicted as quite thick, and even radially 

 striated. It may be very difficult to distinguish the kinetoplast from 

 the nucleus, the two structures often lying very close together or in com- 

 plete apposition. The process of encystment in Herpetofnonas muscarutn 

 was described by Prowazek (1904), and by Minchin (1908) in H. grayi 

 (Fig. 173, 10-16). In the latter the cysts, which were called " Schleim- 

 cysten " by Prowazek, commence to form around the blunt posterior 

 end of the flagellate, which is still in the elongate flagellate state. As 

 the cyst forms, the organism becomes more and more retracted and the 

 flagellum withdrawn, till finally it becomes a pear-shaped structure in 

 which the flagellum is represented only by the short axoneme in the cyto- 

 plasm. The cyst then closes round the more pointed anterior end. It 

 is at first of a gelatinous nature, and encloses a cytoplasm in which the 

 nucleus and kinetoplast can be distinguished. The axoneme finally 

 disappears, leaving in its place an area which stains red with Giemsa stain, 

 and this in its turn vanishes also. The nucleus and kinetoplast become 

 broken up into separate granules, so that their identity is difficult to make 

 out unless the more deeply staining ones are derived from the kinetoplast. 

 The cyst, at first pear-shaped, becomes more circular in outline, the more 

 or less spherical condition being the final one. There seems, however, 

 to be some doubt as to the nature of the structures called cysts in the 

 case of H. grayi. Koch (1906) suggested that the flagellate was possibly 

 the developmental form of the crocodile trypanosome, while Minchin 

 (1908) thought it possibly represented a bird trypanosome. Kleine 

 (1919a) claims that the flagellate is actually derived from the crocodile 

 trypanosome, as tsetse flies bred in the laboratory acquire an infection 

 when fed upon crocodiles harbouring this trypanosome; while Lloyd, 

 Johnson, Young, and Morrison (1924) have produced evidence that it 

 may be derived from either the crocodile, monitor, or toad. If, then, 

 the so-called cysts are actually true cysts, and not merely leishmania 



