BEARING HELMINTHES FEEDING EXPERIMENTS 121 



Where it is desired to obtain intermediate encysted living stages 

 from certain hosts, the microscopic examination of the excreta is 

 naturally useless. Infection is never proved by this means and other 

 measures are successful in a small proportion of cases only. Thus, 

 trichinosis may be diagnosed by examining under the microscope 

 small portions of the muscular structure taken from the living animal. 

 The results are here, to a large extent, dependent upon chance, 

 although the probabilities of success will be increased by frequent 

 repetition of the tests, or by the finding of stages in process of 

 transformation. 



It must not be forgotten that, when handling Helminthes which 

 are parasitic in man, there is considerable danger of self-infection 

 unless suitable precautions are employed. Moreover, the beginner is 

 earnestly warned against all experiments with man as a host. These 

 should be left to experienced scientists who are in a position to guard 

 against the risks inseparable from such undertakings. 



In the great majority of cases, infection with Helminthes takes 

 place by the mouth, though in a few isolated instances it may occur 

 by way of the skin (Ankylostomum strong yloides). In cases where 

 the life-history of the parasite is already known, the host-species into 

 which it should be introduced is readily ascertainable. This applies 

 also to those parasites (many Nematodes) whose history does not 

 include an intermediate infective stage. But where the intermediate 

 stages, or the hosts in which such stages develop, are not known or 

 where these are known, while the definitive host is unknown the 

 finding of an animal of suitable species in which to carry on the 

 experiment is attended by difficulties which are frequently insur- 

 mountable. In cases such as these, we are forced to fall back on the 

 results of previous experiment and, after duly considering all the 

 species which, by their life-history and nutritional habits, might serve 

 as intermediate or definitive hosts to the parasite under consideration, 

 to select the one for experiment which appears to offer some likelihood 

 of success. The conclusive test is furnished by the results of the 

 experiment. Occasionally, however, the introduction of parasites 

 into a host of suitable species is unattended by success. Failure in 

 such cases is due to some technical error, or to changes, the signi- 

 ficance of which has not been understood, in the nature of the infective 

 material itself. 1 



1 Thus R. Leuckart was unsuccessful in infecting himself with Ascaris lumbricoides 

 by means of eggs containing embryos. This led to the erroneous supposition that 

 A. lumbricoides required the agency of an intermediate host before attaining its final 

 development in man. The failure was due, however, to quite another cause. The 

 eggs which were used for experiment had lost their albuminous envelope, with the 

 result that, on reaching the stomach, the embryos which they contained died. The 

 observations of other authors show that, had the albuminous envelope been perfect, 

 infection would most certainly have resulted. 



