ON THE ANATOMY OF ECHINOCOCCUS VETEKINORUM 205 



to circumstances. The neck of attachment of the secondary cysts 

 gradually narrows (fig. 4), and at last the secondary cyst, whose size 

 depends entirely upon the number of Echinococci developed under the 

 endocyst at one spot, is detached and falls into the cavity. So long 

 as the secondary cyst remains attached, its external Echinococci have 

 the normal clear appearance, and are in full health ; but when once 

 it is separated, they appear rapidly to wither away and become 

 yellow, losing their hooks and their corpuscles, and eventually dis- 

 appearing. The original point of attachment of the sac remains as an 

 obtuse cicatrice. 



Von Siebold, who has beautifully described the development of the 

 secondary cysts, has, I think (vide infra), mistaken the one mode of 

 development of the Echinococci outside the endocyst for the only 

 mode. He appears to have seen the endocyst, when he describes 

 the " delicate membrane in which the young Echinococcus-\\cdt.ds are 

 enclosed," and to assume merely, that this membrane bursts and sets 

 the Echinococctcs free upon the inner surface of the parent cyst. Un- 

 derstanding the mode of development to be as stated above, it is easy 

 to comprehend how it is that the Echinococci are so nearly at the 

 same stage of development in all the secondary cysts ; and that this 

 stage has no relation to the size of the cysts. The existence of the 

 external Echinococci upon the secondary vesicles in this way also, 

 becomes not only intelligible, but almost necessary. 



II. The theory which I have to offer of the nature of the Echino- 

 coccus, is based upon three facts which are now well established. 

 1st. That young Cestoid Worms, which, from some cause or other, 

 have passed into any other part of the organism of the animal upon 

 which they are parasitic, than the intestine, become abnormally 

 dilated, at their posterior extremity ; and the anterior end may be 

 retracted into the sac thus formed, which then invests it like a double 

 serous sac — a structureless investment may be excreted round this 

 encysted worm or it may not. Such an altered Cestoid Worm as this 

 is called a Cysticerciis. 



2ndly. A dilated Cestoid worm, such as has been just described, 

 may develope new " heads " with suckers and hooks all over its outer 

 surface, never developing any upon its inner surface. Such a Cestoid 

 worm is the Camints cerebralis. 



3rdly. The Cestoid worms all possess the power of gemmation (or 

 it may be called fission) in their unaltered state: and Bendz (Isis, 

 1844) has distinctly shown that the vesicular extremity of the Cysti- 

 cerciis gemmates. Processes are formed and thrown off, and these 

 develope appropriate heads and hooks, becoming complete Cysticerct. 



