PRIMITIVE EGGS. 321 



to the middle layer. Yet this is apparently a necessary consequence 

 of the fact that the middle layer of the jointed tape-worms passes 

 continuously, as we have seen, throughout the whole length of the body. 



Consequently I see not the slightest ground for regarding the yolk- 

 gland of the Bothriocephali as quite different from that of the Tgeniadae, 

 nor for describing it by another name, which almost necessarily leads 

 to a different conception of it. 



But without supposing a different nature of the yolk-glands, there 

 are considerable differences between the Tseniadse and the other 

 Cestodes in the structure of the sexual organs. These find their 

 expression in the nature of the eggs, which originate from the opera- 

 tion of the germ-glands. In the Bothriocephali they are at first of 

 a relatively large size, and are composed of a thin shell, generally 

 of oval shape, which encloses the ovarian egg, surrounded by the 

 granular yolk-cells. In contrast to these, the generative products of 

 the Tseniadse, which are first fully formed in the end of the uterus, 

 consist of very small round balls with a clear and almost non-granular 

 yolk, and with only a thin and loose covering, which in many of the 

 larger species is prolonged at one (Tcenia marginata) or at both (T. 

 saginata) ends into a kind of tail. 



FIG. 171. Egg of Eothrio- FIG. 172. Recently formed egg of Tcenia 



cephalus latus, showing yolk- marginata (A, B) and T. elliptica (C). 

 cells and shell. ( x 300.) ( x 600.) 



But if we wish to see the latter in their above depicted primitive 

 form, it is necessary to observe the young, or, as they are generally 



FIG. 173. Embryo containing egg, A, of Tcenia solium (without 

 yolk-skin), , of T. nymphcea. ( x 400. ) 



called, immature joints, in which the uterus is still of its original 

 shape, or in which the lateral branches are just beginning to be formed. 

 In the older and so-called mature joints a peculiar alteration has taken 



x 



