DURATION OF THE CAPACITY FOE DEVELOPMENT. 337 



objection — formerly often urged — that an essentially new factor in 

 the life-history of the animal was introdiTced by such experiments. In 

 reality, the experiment is only a methodical repetition of natural pro- 

 cesses. Just as the animals experimented on were in this case com- 

 pelled to swallow a tape-worm joint, so they do unconsciously and by 

 chance in the unconstrained course of nature. The proglottides gain 

 the exterior occasionally by spontaneous effort, but usually along with 

 the fseces ; they are capable of some motion ; they represent, as we 

 have seen, not joints, but independent animals. In warm, damp en- 

 vironnient they can retain their motion and life even for days. They 

 leave the filthy surroundings in which they first find themselves, and 

 seek another abode. Like snails they creep along the ground, climb- 

 ing up a stalk of grass or shrub, and waiting there till eaten by some 

 animal or other. In the dung-eating swine, &c., the transmission is 

 effected directly from the dung. If the new host afford the requisite 

 environment, then the embryos lose their egg-shells and begin their 

 internal wanderings and metamorphoses. 



The characteristic and advantage of experiment, as opposed to obser- 

 vation of the natural process, lies in the choice of the host and the 

 control of the amount introduced. I will not indeed assert that 

 the young brood of the tape- worms always reach their host surrounded 

 by their living parent covering. Apart from those cases (Bothriadse) 

 in which the eggs are liberated and reach the exterior by themselves, 

 the same result might easily follow from the death or rupture of the 

 expelled proglottis. In favourable conditions these isolated eggs 

 remain for long capable of further development. I have been able to 

 infect rabbits with eggs of Tcenia serrata, which had (in September) 

 been kept twelve days in water, and similarly, EoU in Vienna, has 

 had successful results with proglottides which has lain ten days in 

 the open air, and which had even begun to get covered with mould. 

 It is not yet possible to tell how long this potency can be retained ; 

 the period will doubtless vary with the environment. While Gerlach^ 

 was able to infect a pig with decaying proglottides of Tcenia solium 

 five weeks old, in an experiment 1 made, the eggs of Tcenia ccenurus 

 had lost potency after lying in water for eight weeks. ^ This follows 

 more rapidly when the eggs are kept dry. Haubner reports having 

 ineffectually fed a sheep with tape-worm eggs which had been kept dry 

 for fourteen and for twenty-four days, and I have had similar experi- 

 ence in which eggs exposed to an August sun had lost their power 

 of germinating after four-and-twenty hours. 



' Zweiter Jakresber. d. Tcgl. Thierarzneischule, p. 66, Hanover, 1869. 

 ' Davaine, on the other hand, asserts that he has kept eggs of Tcenia solium and Tcenia 

 serrata in water for years, living and unaltered. {M^m. Soo. hiolog., t. iii., p. 272, 1862.) 



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