CESTOIDEA. 13 



substance, similar in consistence to coagulated white 

 of egg, not possessing any elementary granulations, 

 fibres, or cells, and arranged in fine strati£ed layers, 

 of extreme tenuity ; it produces, by gemmation, either 

 upon its external or internal surface, or in its sub- 

 stance, similar vesicles, whicli attain to a variable 

 size, and in their turn reproduce others in a corre- 

 sponding manner ; the parent hydatid, and at a 

 later period, the offspring, undergo considerable 

 changes, lose their fluid, and become reduced to the 

 state of flattened, dried-up membranes. 



The hydatid vesicle, during its development, gives 

 origin by its internal surface to a membrane (the 

 germinal onembrane), which forms an internal invest- 

 ment, and is composed of a fibrillary layer, infiltrated 

 with elementary granules, and altogether different 

 from the hydatid tissue. The germinal membrane is 

 more or less apparent in certain parts of the hydatid 

 vesicle ; it adlieres feebly to the wall of the vesicle, 

 fi:om which it is readily detached, when it dries up, 

 and disappears some time before the hydatid. It is 

 not found in all hydatids. 



Second Phase of Development ; Echinococcus. 



The body of tliis entozoon is oblong or irregularly 

 oval, scarcely visible to the naked eye, and divided 

 into two parts by a circular contraction ; the anterior 

 part forms a head or scolex, furnished with a rostel- 

 lum, which is armed with a double circlet of hooks, 

 and with four contractile muscular suckers ; niunerous 

 calcareous corpuscles are scattered about throughout 

 its substance. 



In the majority of instances the head is in vagi- 



