THE MIG11ATIOX.S OF EXTOZOA. 173 



in 1R5S, tills was the case in almost every instance. If the 

 tapeworms find themselves so well off in the intestine, Avhy 

 shonld the larvie of some of them quit that locality, and he 

 ohli^ed to take a compulsory journey to the hrain? 



The importance of this question, as regards agriculture, 

 has not escaped M. Le Roy, Prefet of the Seine-Inferieure ; 

 and by him w^e have been instigated to experiment on a large 

 scale on this grave subject. 



Several causes have evidently contributed to throw a 

 degree of uncertainty on the results of experiments of this 

 kind. In the first place, must be placed the natural frequency 

 of the Entozoa which are employed upon the animals to 

 which we profess to communicate them. We may notice, 

 also, the circumstance that certain physiologists are accus- 

 tomed to administer worms at several doses, and at more or 

 less distant intervals — a course which allows of all kinds of 

 interpretations. Lastly, we must not leave out of account the 

 unsuccessful results, which have not always been recorded. 



But, let us not delay with these logical considerations ; let 

 ns see what experiment teaches, which is alone competent to 

 pronounce a positive judgment. 



We will, once for all, state that we have taken the greatest 

 precaution to induce precision in our experiments. Thus, 

 when we have sought to implant Canitri of the sheep into 

 other animals, we have not been content simply to ad- 

 minister them e?i masse, as has been done by various 

 experimenters. In order to obtain accurate results, we have 

 determined, on every occasion, the number of the heads or 

 scolices which have been given, by which means w\'3 have 

 been able to decide, with unusual precision, with resroct to 

 certain results which, in any other way of proceeding, miglit 

 have led us to erroneous conclusions. Again, whenever we 

 have made use of these same scolices, we have taken pains 

 to assure ourselves that their development was as far ad- 

 vanced as possible, and that they were actually alive. 



Physiologists have grievously erred in not giving com- 

 parative tables, showing the length of their experiments, and 

 the size of the Entozoa which they have found. The con- 

 sequence is, that we sometimes observe inexplicable dif- 

 ferences in the length of the Entozoa found on inspection 

 after death. 



In a dog wdiich had been made to swallow some Canuri 

 sixteen days before, we found a certain number of Tanice 

 not more than 2 millimetres long, whilst others were 20. 

 After a similar interval of time, an experimenter even 

 obtained some Tcenice which had reached the length of 80 



