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-larvæ, Gammari, Entomostraca, &c.) to undergo the necessary changes. Practically, no doubt, it comes to the same thing in the end. Even if we suppose that the _Ascaris suilla_ and _A. lumbricoides_ are not identical species, still it is evident that any person discharging the eggs of lumbricoids in the vicinity of open waters becomes, by that fact, a source and centre of infection. To ensure an endemic it is probably only further necessary that the human inhabitants should employ the contaminated water for domestic purposes. But time and an increase of temperature must be allowed for the bringing about of those known and unknown larval changes that alike form the necessary antecedents of infection. In this connection I will only add, that if the present position of the question be such as I have here represented it to be, we see that Mosler was not far wrong when he suggested that “contamination of the drinking water with the eggs out of privies is to be blamed” as a source of infection. According to Heller, from whom I quote, Mosler actually demonstrated the presence of the eggs in water thus exposed. In like manner it becomes obvious that Davaine’s practical remark (although it was based on the assumption of a direct infection by the eggs), that filtration will probably be sufficient to prevent infection, loses nothing of its hygienic value.