MODE OF DEVELOPMENT OF THE HEADS. 609 



the buds freely projecting inwards, one always finds others, and 

 occasionally the majority, in which the head-rudiments hang from 

 the brood -capsules into the common bladder-cavity, just like the 

 head-processes of the Cysticerci. They undergo a metamorphosis just 

 like the latter, and differ only in the fact that there is never a recep- 

 tacular sac in which the head is bent together. 



The analogy here referred to corroborates my opinion that the 

 exogenous development of hollow buds is the typical mode of origin 

 for the Echinococcus-'he.a.A.s, while the previously described endogenous 

 mode, even if as frequent as Naunyn^ and Easmussen assert, is in 

 reality exceptional. At any rate, these two observers agree in this — 

 and this is the main point — that both modes of development occur in 

 the Echinococci. 



When the Echinococcus is developed, in the way last described, to 

 that stage in which the histological differentiation begins, then its 

 former attitude changes, and it becomes invaginated from the point of 

 insertion into the internal cavity of the brood-capsule. What took 

 place in the first mode at an earlier stage here occurs at a later 

 stage, with the further difference that the invagination is usually 

 restricted to the posterior (basal) half of the head. The inception 

 of the hooked apparatus and of the suckers in such cases is thus 

 no secondary state, but an occurrence which arises in the original 

 iormation, and is therefore to be regarded as typical. When the 

 invagination of the basal half has taken place, then, as we have 

 formerly described, the now contiguous walls of the hollow bud fuse 

 with one another. The head solidifies and constricts at its point of 

 insertion; in other words, it assumes the appearance of the endo- 

 genously developed head. 



The older the brood-capsule becomes, the more does the number of 

 the inmates increase, so that I have sometimes counted twelve to 

 fifteen, and Eschricht twenty-two. Even in such populous capsules 

 one often sees another new crop of buds (often three or four) at 

 different stages of development. But not only do the heads of the 

 Echinococcus gradually increase, but the number of the brood-capsules 

 likewise, so that the larger and older worms sometimes contain many 

 thousands. 



^ Naunyn notes {loc. cit., p. 621, note) that in those Echinocoocus-hladdeis which were 

 wanned to about 35° C. before being opened, the buds were ahnost always found inside the 

 brood-capsules, while in the same, after cooling, the buds were very frequently found on the 

 outer surface. This would seem to show that the buds can be invaginated or evaginated 

 according to circumstances — that the invagination is, in other words, by no means perma- 

 nent. But this would, I think, also hold good on my theory ; for why should the buds 

 have the power of evagination and an internal cavity, if they develop from first inside the 

 brood-capsule 1 



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