ANIMAL PARASITES. 57 



oval through a spindle-shaped form, to the complete cellular 

 fibre. 2. Plastic fibres, partly still in the form of broad plastic 

 striae, partly as very fine fibrillse, repeatedly crossed, but at the 

 same time always attaining a considerable length. 3. Elastic 

 tissue in all phases of development, from the so-called liga- 

 mentous corpuscles of Virchow, up to fully developed elastic 

 fibres (Luschka). For my own part I have never been able 

 clearly to detect epithelium. Of the secretory cells of these 

 serous cysts and their product, I have already spoken in the pre- 

 ceding pages (PI. I, fig. 10). 



"When, however, the brood of a cestoid worm gets freely into 

 oerous cavities, it either never attains to this formation of an 

 enveloping cyst, as already remarked, or does so at a rather 

 late period, and at a time when the young embryo has become 

 a vesicle of very considerable size, and often exhibits the 

 Tsenia-head in an advanced state of development. At any rate, 

 in these cases, and in places where the young cystic worm again 

 falls into repose, a fresh exudation of plastic masses adapted to 

 the formation of cysts, takes place. The walls of the cyst, more- 

 over, are rich in blood-vessels. 



Having disposed of this subject, we turn, without further 

 interruption to the consideration of the subsequent development 

 of the brood. 



It might have been quite impossible to prove definitively the 

 metamorphosis of the brood, had not Stein succeeded in dis- 

 covering the six booklets of the young embryo, together with the 

 armature of the head, upon the embryos when increasing in size 

 by the reception of fluid nutriment, and also on the cestoid 

 vesicle when developed to the vesicular stage. This has been 

 done with perfect certainty hitherto only in the Cestodea of the 

 cold-blooded animals, and once, although doubtfully, by Leuckart 

 in Cysticercus pisiformis . For the history of the earliest days of 

 the development of the Cestodea, therefore, we must refer espe- 

 cially to those of the cold-blooded animals, and of these the 

 following may be said. By the reception of the secretion of the 

 serous membranes the young Cestoid vesicle swells up. At its 

 anterior end, and therefore at the point where the six embryonal 

 hooklets are situated, a funnel-shaped pit is probably formed 

 first of all, and this gradually penetrates more deeply into the 

 parenchyma of the embryonal body. In the bottom of this pit 

 the first traces of the head then make their appearance, whilst 



