246 PARASITES OF MAN 



simple fresh water, and found them during warm weather 

 escaping before the expiration of five months. I also succeeded 

 in rearing these larvae in pond mud, noticing, at the same 

 time, that after their escape from the shell they grew more or 

 less rapidly up to a certain point, after which they ceased 

 growing. The addition of horses' dung to soft wet mud in one 

 case, and of cows' dung in another, neither appeared to advance 

 nor retard the process of embryonal formation, so long as the 

 embryos were enclosed in their shells. On the other hand, 

 when I reared embryos in simple horse-dung purposely kept 

 moist, they attained a higher degree of organisation than did 

 those in wet mud or water. Having watched hundreds of 

 these larvae under varying conditions, I came to the conclusion 

 that, after escape from the egg, their activity, growth, and 

 strength was most marked when they occupied media which 

 happened to be impure. Davaine experimented on cows, and 

 Leuckart also experimented on horses, with the eggs of this 

 worm without success. Leuckart also failed to rear the larvae 

 in intermediary hosts. Some eggs passed through the water- 

 palmer unaltered. 



These results, so far as they go, seem to be borne out by 

 facts of a professional order. Thus, an instance has been 

 brought under my notice where a considerable number of 

 peasants and their children, dwelling in a parish in Yorkshire, 

 were infested with this worm. There was, in short, a local 

 endemic helminthiasis. Through the parish runs a stream 

 which supplies the cottagers with all the water they employ for 

 domestic purposes (washing, drinking, and so forth). Some of 

 the peasants living by the side of the stream keep pigs, and 

 the sewage from this source has been allowed to pass into the 

 stream itself. Now, if Schneider's determination as to the 

 identity of the lumbricoid of man and the pig is correct (which 

 I do not doubt), the explanation of the cause of the endemic 

 becomes a very simple matter. But it does not explain all 

 that we desire to know about the young worms. Either the 

 freed embryos before they enter the human bearer accomplish 

 further changes of form and growth in the sewage or impure 

 water ; or, what is far less probable, they pass into the bodies 

 of intermediary hosts (such as insect -larvae, Gammari, Ento- 

 mostraca, &c.) to undergo the necessary changes. Practically, 

 no doubt, it conies to the same thing in the end. Even if we 

 suppose that the Ascaris suilla and A. lumbricoides are not 



