DEVELOPMENT OF EMBRYO. 69 



Even in its structure it differs essentially from the other ordinary 

 embryonal vesicles. In the Echinococci, the epidermis of the 

 embryonal vesicle adheres quite firmly to the enveloping cyst, 

 and forms several concentric layers. The muscular layer which 

 usually follows upon this is wanting, and instead of it and the 

 true interstitial and medullary layers, there follows a layer of a 

 granular vesicular structure, with vessels and calcareous cor- 

 puscles. It is probable that the villous, obconical elevations or 

 thickenings, which occur on the innermost vascular layer, become 

 converted into new vesicles or brood-capsules, which are, of 

 course, destitute of the six small embryonal booklets, and which 

 produce the individual scolices, but which are seldom directly 

 developed into scolices, united for some time with the mother- 

 vesicle by a stalk through which pass 2 4 vascular stems, con- 

 nected with the vascular system of the head. Even the individual 

 scolices are probably in general produced in the interior of such 

 capsules; and from the preceding statements it is not difficult on 

 the whole to understand this process, as the scolex-formation com- 

 mences in accordance with the type of the Ccenuri and Cysticerci 

 by the production of cephalic processes in the interior of the 

 separate brood-capsules. This formation of cephalic processes is 

 not, however, effected in a place produced by the inversion of the 

 mother-vesicle, which may be reached from the outer wall by 

 means of the above-mentioned perforation, but on the peduncu- 

 lated vesicle, and indeed on the end of this vesicle which stands 

 opposite to the peduncle. Thus, there only appears to be the 

 following distinction between the two principal forms .of Echino- 

 cocci above-mentioned. In Echinococcus scolicipariens the brood- 

 capsule remains adhering to the mother-vesicle by its peduncle 

 and proliferates in this way; in Echinococcus altricipariens the 

 brood-capsule, with its peduncle, separates from the mother- 

 vesicle, becomes thickened in its walls like the latter, and then 

 proliferates independently. In the latter species both forms of 

 production go on together in one mother-vesicle, and it also 

 happens that brood-capsules separate not with, but from the 

 peduncle, when their margins roll round, and the brood projects 

 outwardly in groups into the fluid of the mother-vesicle, the 

 everted inner surface becoming the outer surface, or any separated 

 brood which may occur free in the daughter-vesicles, is emptied 

 out into the fluid of the mother-vesicles. A single, separated 

 scolex can hardly ever become converted into a brood- capsule 



