368 



FAMILY: TRYPANOSOMID^ 



culture experiments of Drbohlav (1925), referred to above (p. 364), appear 

 to be conclusive. 



By a gradual shortening of the body of both the leptomonas and 

 trypanosome forms, smaller stumpy individuals are produced, and 

 these become attached to the lining epithelium of the hind-gut. Repro- 

 duction of all these free and attached forms takes place by longitudinal 

 fission, often producing enormous infections of the gut. Encystment 

 takes place by a gradual shrinkage of the body, or in some of the 

 trypanosome individuals by the doubling of the body into a U, the space 

 between the limbs of which gradually fill in, so that the axoneme follows 

 a characteristic curved course in the cytoplasm. There are three methods 



Fig. 171. — Three Methods by which the rounding-up (Encystment) of Ilerpe- 

 tomonas muscarum takes place in the Hind-Gut of the House Fly. (After 

 Wen YON, 1913.) 



1-3. Retraction of leptomonas form. 



4-8. Rounding up of trypanosome form by looping of the body. 

 9-11. Rounding -ujj of trypanosome form by retraction of body. 



by which retraction of the body and encystment may take place (Fig. 

 171). Becker (1923c) thinks that encystment of H. muscarum always 

 takes place after the trypanosome type has been developed. The cysts 

 appear to have a definite cyst wall. Their function is undoubtedly the 

 transmission of infection from fly to fly, but, as Patton (19106) and Becker 

 (1923c) have shown, flies may be infected by ingestion of adult flagellates 

 or the pre-encysting forms passed in the faeces, as well as by cysts. The 

 cysts would probably ensure protection against a period of desiccation. 



From his earlier work, Patton concluded that, though the larvae of 

 Musca nehulo might be infected with flagellates, these did not appear to 

 survive the pupal stage, as flies hatched from infected larvae were free 

 from infection. In a later paper (1921) he appears to have modified this 

 view, for he states that infections will pass through the pupae to the 

 adults. Becker (1923c) was unable to detect larval infections. As 



