MSCOVEEIES OF VON SIEBOLD, DUJARMN, AND VAN BENEDEN. 37 



such as that the germ of the tape- worm had " lost its way " — that is, 

 arrived at some place that was not suitable to its requirements. "We 

 shall have to examine this theory of " straying " later on, but for the 

 present it may be remarked that von Siebold believed it to be of very 

 wide application, and to explain the existence of many other sexless 

 worms (even Trichina), which had not come to a full development on 

 account of having strayed into unsuitable localities. 



Later von Siebold made the developmental history of tape-worms 

 the subject of a special memoir,^ in which he sought to prove, with 

 special reference to Tetrarhynckus, that the tape-worm heads found 

 so abundantly encysted in predatory fish, originated from embryos 

 that had wandered there, and that these developed into the sexual 

 adult by the formation of segments (Figs. 24 and 25), when their 

 host was swallowed by some other carnivorous fish in whose ali- 



FiG. 25. 



Fig. 24. 



Figs. 24 and 25. — Echiiiohotkrium minimum 

 (after van Beneden), isolated living head and 

 tape-worm. 



Fig. 26. — Transformation of the 

 bladder-worm into a tape-worm 

 {Twnia sen-ata). 



mentary canal these chains of tape-worm segments were formed. 

 Von Siebold based these arguments upon induction, but their cor- 

 rectness was subsequently certified by a direct proof of the metamor- 

 phosis and migration of these tape-worm heads. 



Contemporaneously with von Siebold, or even earlier, van 



Zei«4e/«%,-/«*^iap.hZp(rf>i,-J84,<iw*»i98, 1850. 



