52 ANIMAL PARASITES. 



us with an important support for the assumption of a new active 

 migration of the brood from the capillaries. 



If the Cysticercus bears no embryonal hooklets, and loses them 

 in the current of the blood, as a matter of course it cannot escape 

 from the blood-vessels unless the vessel in which it is seated 

 bursts at last. At any rate, the fact that the embryos still 

 possess their hooklets at the commencement of the vena porta 

 does not prove that these hooklets are not subsequently lost by the 

 friction of the stream of the blood in the narrower vessels, and 

 then these bookless creatures would remain seated in the interior 

 of the vessels. As regards those Cysticerci or Echinococci in the 

 walls of which the histological constituents of vascular tubes are 

 recognised, no one will doubt that we have to do with embryos 

 which have remained sealed in the interior of the vascular tubes; 

 but when these indications are wanting (and this is certainly the 

 general rule, with extremely few exceptions), we must suppose 

 that there is a further emigration out of the vessels, either 

 indirect and secondary, or primary and direct ; the latter, how- 

 ever, especially, if the six hooklets still exist upon the young 

 vesicular worm. 



To sum up the whole briefly, the migration of the young of 

 the Cestodea to the places where we meet with them as vesicular 

 worms, or in analogous states, takes place in the following way : 



1. A portion of the six-hooked brood in all species of Cestodea 

 (whether they take up their abode in cold- or warm-blooded 

 animals, whether they possess small or large embryonal hooklets, 

 and whether they do or do not pass through a true vesicular 

 state,) may reach their dwelling-place directly and by active 

 migration. 



2. Another portion, after a longer or shorter active migration, 

 reaches the vascular system of the new host (the blood-vessels, and 

 perhaps also the lymphatic system, as, according to Virchow, is 

 the case with the Echinococci), is subjected to a passive migration 

 here with the fluid, remains fixed in the smallest ramifications, 

 and — 



a. Becomes further developed in the vascular tube, making use 

 of the walls of the vessel as an envelope (cyst), so as to remain 

 there permanently, or — 



b. Migrates passively into the neighbouring tissues, after the 

 rupture of the walls of the vessel, in consequence of the swelling of 

 the body of the embryo, or — 



