32 THEORY OF THE ORIGIN OF PARASITES. 



The investigations of von Siebold were not confined to the eggs of 

 Trematodes, but were extended to the eggs of other intestinal worms, 

 and led to the important discovery that in the tape-worms also the egg 

 contained an embyro before it was laid. Here also the embryo was 

 totally different from the parent, a simple spherical mass, dis- 

 tinguished only by the possession of six stylet-shaped hooks (Fig. 20) 

 arranged in pairs at the anterior pole of the body, and capable of being 

 moved like levers. ^ The subsequent changes undergone by this embryo 

 were for some time uncertain, though there was no doubt that they 

 could only pass into the fully formed animal " by a kind of meta- 

 morphosis." 



Fig. 20. — Eggs of the tape-worm with six-hooked embryo. 



Whether von Siebold perceived at that time the important bearings 

 of his observations, must be left undecided. In any case, he neglected 

 to follow them up to their legitimate consequences. This was done 

 some years later by Eschricht,^ who fully discussed the question of 

 the origin of Entozoa for the first time since the days of Bremser, 

 and who had also, in his masterly researches upon BothriocepTialus 

 latus,^ decidedly opposed the idea of spontaneous generation. In this 

 work Eschricht collected all the facts that had been lately discovered 

 about the metamorphosis of intestinal worms, and endeavoured to 

 support the view that these phenomena were commonly found among 

 the Helminths. He adduced the great development of the generative 

 organs and the fertility of the Entozoa (the number of eggs produced 

 annually by a single Bothriocephalus latus must be reckoned at at least a 

 million, and of a female thread-worm at 64,000,000 ! ) as evidence 

 that did away with the enormous difficulties besetting the theory of a 

 transmission to " suitable localities." Finally, he recalled the fact, first 

 discovered by Abildgaard,* and also known to Bremser and Eudolphi, 



' Burdaoh, " Fhysiologie," !oc. dt. Previously to von Siebold, Goze had seen these 

 embryos, but his description and figures {" Versuch, &c." tab. xxii., figs. 20-22,) are so 

 insufficient, and for the most part so incorrect, that no conclusions can be drawn from them. 



" Edin. Neio Phil. Jovmal, 1841. 



' Nova Acta Acad. Cms. Leap., t. xix., suppl. 2, 1841. 



* Naturhistorish Selsh Shrifler, Bd. i., p. 53, 1790, ; see the remarks made on p. 24. 



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