226 THE GROUSE IN HEALTH AND IN DISEASE 



Others will crawl out of the thin edge of the culture medium and become stranded 

 on the dry glass. This metamorphosis takes place between the eighth and 

 sixteenth day from the commencement of the culture, the difference in time 

 depending almost entirely on the temperature at which the culture is kept. 



If the blotting paper be now removed, and the upper part of the Petri 

 dish be put aside, so that the moisture on its inner surface, which contains 

 The second tae actively wriggling metamorphosed larvae, be allowed to evaporate 

 encyst- slowly, it will be noticed that as the water disappears the move- 

 ment." ments of the larvae gradually diminish and eventually entirely cease, 

 so that ultimately the larvae lie sometimes making irregular figures like notes 

 of interrogation, sometimes coiled up like a watch spring (Fig. 29). If drying 

 proceeds sufficiently slowly it would be found on examining the dish with a 

 hand lens, that when all traces of moisture have disappeared the little coiled 

 larvae stand out as turgid, glistening streaks. They seem to be capable in this 

 condition of retaining a certain amount of moisture within their thick resistant 

 cuticle for several days, and to make up for any loss of fluid by evaporation by 

 slowly retracting the body from either end and of detaching themselves 

 from their cuticular skin (Figs. 26, 27, 30). This retraction may go on to such 

 an extent that if one suddenly adds water once more to a Petri dish containing 

 such dried forms the little worms are found enclosed in long sheaths that extend 

 much beyond each end, recalling the sheathed embryos of filaria seen occasionally 

 in the blood of man. 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 (Fig. 30). It appears 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 development. The first is completed prior 

 to metamorphosis ; the second, subsequent thereto, is not completed during the 

 non-parasitic period. 



So much then for artificial experiments. 



The following details of an experiment made during August 1909 serve 

 to illustrate what actually becomes of the hatched worms under 



Larval . 1-1 



migrations natural conditions upon the moors. A culture made m the manner 



described above was taken to a small village, on the coast of the Bay of 



Cardigan, where no Grouse lived or had been known to exist for many years. 



