66 LIFE-HISTOKY OY PARASITES. 



animals, e.g. fishes, may be occasionally deceived, and devour tape- 

 worms under the delusion that they are nutritious food. It is, 

 moreover, evident that the further development of these eggs, when 

 they have reached the body of some animal, is only possible when 

 the conditions are favourable, and when the eggs themselves contain 

 a living embryo. It is not easy to say how long the embryo will 

 retain its vitality; accidental and even constant conditions bring 

 about the greatest variations in this respect. In the eggs of the 

 common thread-worm (Ascaris) I have seen active embryos even after 

 the lapse of two or three years, ^ as also in the eggs of UcMnorhynchus ; 

 whilst, on the contrary, the eggs of tape-worms usually lose their 

 vitality within a few weeks, even when kept damp. 



The eggs first of all, we may suppose, reach, in a living condition, 

 the stomach of their host, where, if the digestive juices be of sufRcient 

 strength, the shell is dissolved ; variations in this respect have been 

 already alluded to (p. 58). The embryo, which was hitherto sufficiently 

 protected by its outer cuticle against dissolution, now becomes free, 

 and acquires the possibility of growth and development. 



DEVELOPMENT OF THE GEEMS AETEE MIGRATION. 



That the embryos of some Entozoa, directly they are hatched, leave 

 the stomach of their host, and find their way into its intestine, where 

 they arrive at sexual maturity, has been placed beyond doubt. I suc- 

 ceeded in infecting a sheep with TricJwcepJialus by feeding it with the 

 eggs containing embryos.^ In a similar fashion, according to Ehlers, 

 hens and other birds are infected with the tracheal parasite Syngavms, 

 and man (according to Zenker and myself) with Oxyuris. Kiichen- 

 meister and Davaine attempted to breed Ascaris lumhricoides from 

 eggs by drinking water containing them, but numerous and careful 

 experimeiits in this direction by Mosler and myself led invariably to 

 a negative result. 



In some cases {e.g., Dochmius trigonocepJialus, as above men- 

 tioned) the free embryos also attain to maturity without change of 

 locality. It is usual, however, for the development of the young 

 parasites, whether hatched in the stomach or outside the body, 



1 Davaine saw embryos alive after four years, and even after five years had elapsed 

 he was able, by heating them, to induce signs of vitality. (MSm. Sec. Biol., t. iv., p. ,261, 

 1862.) He also states that he was able to preserve alive for years eggs and embryos of 

 Tcenia solium and Tcenia serrata. (Ihid., t. iv., p. 273, 1862.) 



^ See Vol, II. The attempt here referred to is the first which has established the 

 continuous development of an intestinal worm. Of course, . Davaine and others had, 

 before this, occasionally asserted such a development, but what they adduced was in no 

 way convincing. 



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