HYDATIDS. 127 



and occasions a peculiar, prolonged trembling motion, 

 which, in some cases, forms a means of diagnosis of 

 this class of tumours. 



The periods during which the hydatids live is 

 uncertain, but it is probably rather long, as even in 

 tumours which are of considerable duration, some of 

 the hydatids may be found to be unaltered ; more 

 frequently, however, they will be found to have 

 undergone some changes. The echinococci which 

 they contain may almost entirely disappear, and the 

 hooklet/S which remain will then furnish the only 

 indication of their previous existence ; the membrane 

 of the hydatid may have lost more or less of its 

 transparency and of its homogeneity, in consequence 

 of the development of apparently fatty granules in 

 its substance ; the membrane may also have become 

 collapsed, but the Hquid which it contains usually 

 preserves its limpidity ; sometimes the membrane is 

 ruptured, and its cavity is then completely effaced. 

 It may also happen that all the hydatids contained 

 within a cyst lose xheir fluid simultaneously ; the 

 vesicles then collapse and are folded together, whilst 

 the cyst undergoes a proportional contraction, until 

 eventually the latter only contains the membranes 

 folded together, like the petals of a poppy enclosed 

 within the calyx. 



The substance of the hydatid resists absorption 

 or total transformation for a very considerable period ; 

 so that even in the oldest hydatid tumours there may 

 be found membraneous shreds and the booklets of the 

 echinococci, which afford evidence of the primitive 

 character of the tumours. 



As has been already mentioned, the hydatids 



