<4 LECTURE IV. 



lateral appendages. The eggs of T. cucumerina and T. crateri- 

 formis are collected in groups of from ten to twenty, in a common 

 gelatinous nidamental capsule. In the eggs with a single brown shell 

 the young are liberated by the fall of a kind of lid from one end.* 

 In the other eggs the tunics dehisce irregularly to liberate the larva. 



Wide and interesting questions on animal development are con- 

 cerned in the solution of the narrower, but not less interesting 

 ones, as to how and where the young of the impregnated ova of tape- 

 worms are developed ? In no instance have they been observed to 

 be excluded in the intestines of the animal in which they were 

 formed. The eggs are cast out with the excrement into the waters 

 or on the surface of the earth. 



Development of the cestoid embryo begins, however, as soon as the 

 proper tunics of the ovum are formed, and it is commonly far 

 advanced before the generative joints of the parent are expelled. 

 The process has been studied by Siebold in the Bothriocephalus pro- 

 boscideus, B. crassicolUs and B. infundibuliformis; in Tceraa solium., 

 and in a score of other species of Tcenia. In each case the embryo, 

 presenting a rounded or oval shape, according to the shape of the 

 ^^^, is characterised by having six spines or booklets, retracted into 

 the interior of the body : one middle pair lying in the axis of the 

 body, the other two lateral pairs diverging and directed outwards. 

 The embryos were found so armed even in the Cestoidea, which, 

 when mature, are devoid of hooks and form the Tcenice inermes of 

 Rudolphi. In the T. cyatliiformis, the two middle hooks are the 

 largest and most bent. In Tcenia porosa^ one of the oblique lateral 

 hooks is very thick, the other very slender. The young of the 

 Bothriocephalus proboscideus present an ovate subdepressed form ; 

 and when they are two-thirds of a line in length, the bothria may be 

 discerned at the larger end. The embryo of Tcenia ocellata, when 

 half a line in length, shows plainly, under the compressor, the four 

 suckers, f 



This discovery was soon confirmed by Dujardin, who gave a 

 figure of the embryo of Tcenia cucumerina, observed by him in the 

 ova of the fully developed generative joints of that tapeworm from 

 the intestines of a dog \ : and he has described the contractions of 

 the body of the larva and the movements of the booklets. 



The earlier phases of development have been well traced by 

 KoUiker, in both Tcenia and Bothriocephalus. \ Development is 

 first indicated by a clear place in the centre of the egg, which becomes 

 more and more clear, largei', and nearer the surface of the yolk, where 



* LX. p. 201., and XX [V. p. 148. t LX. p. 202. 



X LXV, p. 29. pi. i. fig. 10. § LXVI. p. 91. 



