126 ENTOZOA FOUND IN MAN. 



The substance of the hydatid is homogeneous, 

 friable, elastic, destitute of fibres or cells, and ana- 

 logous in its appearance and consistence to the boiled 

 white of an egg. This substance constitutes a mem- 

 brane which is arranged in stratified layers ; these 

 layers, which are extremely fine, are evident, even in 

 the smallest hydatids, upon a microscopical examina- 

 tion, and form a distinctive character of this patho- 

 logical product. 



Although they are smooth and even externally, 

 hydatids often present numerous irregularities on 

 their internal surface. The smallest hydatids are 

 constituted like the largest, as regards their walls ; 

 and if they are not very small, the existence of a 

 central cavity can also be ascertained. The cavity 

 contains a more or less abundant liquid, which is 

 ordinarily serous and limpid. It is lined, in the 

 fertile hydatids, by a special membrane (the germinal 

 membrane), from which the echinococci arise. 



Sometimes several small hydatids are found in 

 company with a large one, to which they are external, 

 and, more frequently still, a large hydatid encloses 

 several smaller ones, which float freely within its 

 cavity ; or numerous small ones may be found adhe- 

 rent to either the external or the internal surface of 

 a large one. These hydatids arise, like buds, from 

 the substance, or on the surfaces, of the envelopes of 

 the parent hydatid, form elevations upon these sur- 

 faces, and after growing and becoming hollow, are 

 finally detached. 



Those hydatids which are much distended by fluid 

 possess remarkable elasticity, so that the least shock 

 to any part is transmitted through the whole mass. 



