116 PRACTICAL PARASITOLOGY 



Abildgaard, intermediate stages maybe traced and the entire life-history 

 of the worm, from the egg to the mature adult, may be followed. 

 Generally speaking, three stages are recognized in the development 

 of Helminthes : the embryo ; the intermediate, young, or immature 

 worm ; and the sexually mature, adult parasite. Of these three 

 stages, the two latter are developed within the bodies of separate 

 hosts, which are known respectively as the "intermediate" and the 

 " definitive" hosts. As the embryo does not usually develop until 

 after the egg is deposited and, frequently, not until after it has left 

 the body of the definitive host, the egg must be hatched out by 

 artificial means. This is usually an easy matter. All that is required 

 is to place the eggs, as soon as possible after they are laid, under 

 conditions favourable to their development. Or eggs may be used 

 which have been removed from the uterus immediately before deposit. 



Eggs which, according to species, have been deposited either before 

 or in the course of cell-division, will develop in water or damp earth ; 

 those of Cestodes and Trematodes in water, those of Nematodes in 

 damp earth or faeces. Thus, the eggs of the broad tapeworm 

 (Dibothriocephalus latus) or of the liver-fluke (Fasciola hepatica) 

 should be removed from the faeces or from the terminal portion of 

 the uterus, put into shallow vessels containing filtered water, covered 

 with a glass plate, and allowed to remain in a well-aired room. The 

 period necessary for the development of the eggs varies with the 

 temperature and the lighting. In summer, development occupies 

 a few weeks only ; in spring and autumn, it takes longer ; while in 

 winter, unless an incubator is used, it is arrested altogether. Cultures 

 of this kind sometimes fail owing to the development, in large quanti- 

 ties, of bacteria or other vegetable or animal parasites. Or it may 

 happen that the eggs are unripe. In the case of the liver-fluke, the 

 latter contingency may be guarded against by using for experiment 

 those eggs only, which have been taken from the gall-bladder of the 

 host. 



Failure is still more frequent in the case of Nematode eggs 

 hatched in damp earth. The material should be placed in watch- 

 glasses, or in shallow glass or porcelain bowls, and carefully covered, 

 without, however, being rendered air-tight. They are then put under 

 an inverted funnel, the stem of which is closed with a wad of cotton- 

 wool. The best medium is common garden mould, which has pre- 

 viously been heated in order to kill all germs and anirnalculae. When 

 cold, it should be moistened with filtered water (though boiled water 

 is better) and it should be kept wet. The eggs of Nematodes may 

 also be developed between wet blotting paper. In the case of many 

 varieties, as, for instance, Trichocephales, the eggs have such an 

 immense power of resistance that they will develop in the reagent 



