SUMMARY. 93 



The envelope then becomes firmer and more membranous; its 

 contents draw together inwards, and become converted into a 

 small, spherical corpuscle, which is surrounded by a separate 

 membrane, and on the outer surface of which (4 — ) 6 small 

 hooks are attached, which afterwards grow inwards to a greater 

 depth. This is the embryo. At the same time, the external 

 envelope is frequently covered with asperities or tubercles, 

 as in our Tanice, and becomes converted into the egg-shell, 

 which is truly not an egg-shell, but an embryonic structure (a 

 sort of amnion). The small vitelline mass remains outside this 

 so-called egg-shell in the form of a crumbly mass, envelopes 

 itself in an albuminous enveloping membrane, or becomes a 

 second, internal envelope, which, however, would be erroneously 

 regarded as a proper egg-envelope. 



Thus we have seen how the segments, with their embryonic 

 structures, are developed, and, when they have become the hind- 

 most in the colony, cast themselves loose. Their destiny has 

 been already described (sections I and II), and thus the subject 

 has been concluded, but which we will again sum up briefly. 

 The developmental history of the Cestoidea is best understood if 

 we consider it from the following points of view : 



I. The mature sexual animal, which is produced by asexual 

 gemmation, separates from the colony as soon as it has attained 

 its maturity, migrates actively from the intestinal canal of its 

 host into free nature, and thence passively into the stomach of 

 another usually herbivorous animal. It bears within it — 



II. The grand-nurses or embryos produced by sexual, and 

 perhaps by asexual reproduction, furnished with four or six 

 hooks, which are destined to enter passively into the stomach of 

 a herbivore, and thence to migrate through the body of the latter, 

 either actively or by the intervention of the vessels, active- 

 passive-actively. (The asexual propagation of the brood in the 

 Cestodea is rendered improbable even by the fact that in the 

 whole developmental series of these Cestodea, a sexually mature 

 animal has never been met with). 



III. The resting scolex (nurse) produced by asexual gemmation 

 in and by the enlarged, six-hooked embryo, still concealed within 

 this embryonal vesicle, or lying beside it enclosed in peculiar 

 cysts, or in closed serous cavities. In its non-gemmiparous part 

 the embryonal vesicle acquires, according to the species of cestoid 



