608 STRUCTURE AND DEVELOPMENT OF THE ECHINOCOCCUS-HEADS. 



the rudimentary head has grown to any considerable size. Since one 

 sometimes sees beside such small invaginated buds others which 

 represent a later developmental stage, it often appears as though the 

 Uchinococcus-heaLds budded off as a whole inside the brood-capsule, 

 and were developed from the first in their subsequent position with 

 the cuticle outwards. At first a simple cylindrical process, the bud 

 gradually assumes a club-like or pyriform shape, becoming constricted 

 at its basal end. Afterwards it becomes modified by the formation 

 of the hooks round about the free rounded end, where the border 

 projects in a sort of circular fashion. Further back, at the bulging 

 portion of the bud, the first traces of the suckers begin to appear 

 (Fig. 327). 



Fig. 326. — Brood-capsule of Eohinococcus 

 veterirwrum, with adult and hollow rudi- 

 mentary heads. ( x 40. ) 



Fio. 327. — Development of the 

 Echinococcus-heads from those hanging 

 freely into the internal cavity of the 

 brood-capsule (after Wagener). ( x 90.) 



The rudimentary hooks appear first in the form of a somewhat 

 thick fringe of prickles, which surround the cephalic end like a girdle, 

 but which afterwards all disappear except the foremost rows, which 

 have grown gradually stronger. When the circlet of hooks has almost 

 attained its subsequent structure, it is retracted along with the 

 adjacent sucker-bearing portion, and sinks into the gradually enlarg- 

 ing posterior portion of the body. The internal cavity of the bud, 

 which has not been lost by the invagination, then becomes obliterated, 

 and the histological differentiation progresses, producing muscle-fibres, 

 vessels, and calcareous corpuscles; and after constriction and the 

 formation of a stalk at the point of insertion, the head has attained 

 its final form. 



I will not deny that the development of the Uchinococcus-he&ds 

 sometimes takes place in the way thus described (after "Wagener). I 

 have indeed sometimes observed JSchinococci in which the rudi- 

 mentary heads almost all projected into the brood-capsules, and 

 exhibited the above developmental stages in uninterrupted succes- 

 sion. But I must most emphatically deny that the development 



of the heads in the above-described position is the only mode. Beside 

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