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 J'olk- 

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

 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 betweel^the Tseniadae 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 Bothriocepliali 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 Tseniadae, 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 {Taenia marginaia) or at both (T. 

 saginata) ends into a kind of tail. 



CS) 



Fig. 171. — Egg of Bothrio- 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 Tama solium (without 

 yolk-skin), B, of 2'. ni/mphcea. ( 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 

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