114 PARASITES OF MAN 



margin of the proglottis rather below the central line ; proscolex 

 or embryo giving rise to the formation of large proliferous 

 vesicles, within which the scolices or echinococcus-heads are 

 developed by gemmation. 



When an animal is fed with the mature proglottides of Ttenia 

 echinococcus the earliest changes that take place are the same 

 as obtain in other cestodes. The segments are digested ; the 

 shells of the ova are dissolved ; the six-hooked embryos 

 escape. The embryos bore their way into the organs of 

 circulation, and thence they transfer themselves to the different 

 organs of the host ; being especially liable to take up their 

 abode in the lungs and liver. Having arrived at this, their 

 resting stage, the embryos are next metamorphosed into 

 hydatids. According to Leuckart's investigations the juvenile 

 hydatid is spherical at the earliest stages ; being surrounded by 

 a capsule of connective tissue formed from the organs of the 

 host. After removal from its capsular covering, the vesicle 

 consists of a thick laminated membrane, forming the so-called 

 cuticular layer, and a central granular mass, which subsequently 

 becomes enveloped by a delicate granular membrane. At the 

 fourth week the echinococcus capsule measures about 53" in 

 diameter, its contained hydatid being little more than half this 

 size. Its future growth is by no means rapid, seeing that at 

 the eighth week the hydatid has attained only the -/' in diameter. 

 At this period the central granular mass develops a number of 

 nucleated cells on the inner surface of the so-called cuticle. 

 These cells, which at first are rounded or oval, become angular 

 or elongated in various directions, and even distinctly stellate ; 

 and in this way a new membrane is formed, constituting the 

 so-called inner membrane or granular layer. The intermediate 

 stages between this condition and that of the fully-formed' 

 echinococcus hydatid have not been satisfactorily traced in 

 detail ; nevertheless, Krabbe and Finsen's experiment on a lamb 

 showed that within a period of little more than three months 

 well-developed echinococcus-heads may be formed in the interior 

 of the vesicles. It is thus clear that the production of scolices 

 immediately follows the formation of the granular layer, and 

 this is succeeded, though not invariably, by the formation of 

 daughter- and grand-daughter-vesicles, which are sometimes 

 termed " nurses." These latter may be developed exogenously 

 or endogenously. 



The appearance of hydatids varies very much according to 



