230 THE GROUSE IN HEALTH AND IN DISEASE 



the blood of man (Fig. 34). This second formation of a sheath, 

 or as it is sometimes called, the " encystment," is the last stage 

 of the development of the larvae outside the body. It appears 



FIG. 32. 



As seen iri water. 



FIG. 33. 

 As seen stranded on sides of dish. 



Larval forms of T. peryracilis. 



FIG. 34. 



Encysted larvae of T. pergracilis. 

 Appearance on addition of water to forms shown in Fig. 33. 



to be a necessary preliminary to the attainment of infectivity, 

 and once this stage is reached the larvae can remain alive without 

 food or further growth for weeks. The larva does not shed 

 this second sheath until it reaches the alimentary canal of the 

 Grouse. There are thus two moults in the extra-corporeal 



