194 EMBRYOLOGY 



equipped with three pairs of hooks. Farthermore, according 

 to VAN Ben EDEN, a cortical layer can early be distinguished 

 from the differently constituted internal cell-mass ; and 

 SCHAUINSLAND also speaks of smaller peripheral cells and 

 larger central ones. It is natural to regard this as a differ- 

 entiation into the two germ-layers, though Schauinsland be- 

 lieves that such is not the case. According to him, the entire 

 ectoderm, with the two membranes, is excluded from f ui^ther 

 participation in the formation of the embryo, which consists 

 exclusively of a homogeneous cell-mass : the entoderm. This 

 point, and especially the origin of the layers of the embryo, 

 appears to us in urgent need of renewed investigation. 



With Schauinsland, we regard the homology of the embryonal mem- 

 branes of the Bothriocephnlidce, Ttcniadce, and Distomidcc as unquestion- 

 able. The different development of the second membrane — in the one 

 case into a ciliated layer, in the other into a chitinous layer — is determined 

 by the mode of life of the particular worms. Some of them inhabit 

 animals which continually come in contact with water. In these the 

 deposited eggs develop very quickly and require no special protection. 

 The others inhabit land animals. Their eggs reach the outside world 

 while still within the proglottis, and the more the already developed 

 embryos are protected against desiccation, the better their prospects for 

 existence. Hence the development of the chitinous membrane. In such 

 Tccniada, on the contrary, as inhabit aquatic animals, the chitinized 

 embryonal membrane may be absent, and in place of it there may 

 appear a thin membrane, similar to the non-ciliated ectodermal mantle 

 of many Bothriocephalidce (Schauinsland, No. 12). 



The further development of the six-hook embryo (Fig. 

 96 A) takes place only after it has migrated into an 

 intermediate host. Either this may take place dii-ectly, — 

 when the embryo, as in the Bothriocephalidce, is a free- 

 swimming larva, and so at once migrates into an aquatic 

 animal, — or the embryos, still enclosed in the egg-membrane, 

 may enter by passive means into the intermediate host. 

 Generally this happens by the segment of the tapeworm, 

 which crawls about on plants, being swallowed with the 

 food. The proglottis is digested in the stomach, the eggs 

 thereby become free, their membrane ruptures, and the 

 embryos now find themselves within the intestinal canal. 

 They do not remain there long, but penetrate into the in- 



