480 ANATOMICAL AND PATHOLOGICAL OBSERVATIONS. 



quantity of cretaceous matter which becomes very much 

 condensed. 



The middle membrane then appears to play the most im- 

 portant part in the economy of the hydatid ; the external 

 membrane acting only as an organ of defence. 



Of this peculiar form of animal three species have been 

 determined, the characters of which are derived from the 

 structure and appearance of the germinal membrane. In 

 AcepJialocystis simplex, the lowest of these forms, the whole 

 structure of the animal is much more homogeneous, trans- 

 parent, and gelatinous, than that of the two higher forms ; the 

 cyst is not divided into separate parts, and the young are 

 developed promiscuously throughout its internal surface. 



In AcepJialocystis armatus, the young are developed from 

 a true germinal membrane, each of the young arising as a 

 separate cell, and afterwards throwing off internally successive 

 broods of young independently. It is also distinguished from 

 the other species by the teeth which it possesses during the 

 period of its attachment to the parent germinal membrane. 

 These teeth are generally exactly opposite the spot of attach- 

 ment, are quite straight, barbless, and form an irregular 

 circlet, somewhat similar to that of Csenurus and Cysticercus. 

 They are lost as soon as the animal leaves the germinal 

 membrane and becomes free, and not the slightest vestige of 

 them can be seen, even upon the shreds of membrane alluded 

 to above, which at one period formed the internal membrane 

 of the parent sac. 



In the Medical Gazette for Nov. 22, 1844, p. 268, there is 

 an abstract of a paper read before the Koyal Medical and 

 Chirurgical Society of London, by Erasmus Wilson, on the 

 " Classification, etc., of Echinococcus hominis" There can be no 

 doubt that the Echinococcus here described by Mr. Wilson, 

 and the AcepJialocystis armatus, are both one and the same 

 species. The bodies described by Mr. Wilson as the 



