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XLV] on numerous experiments on man by Continental observers, and until recently seemed to be justified. Stewart, however, has shown that if ripe ascaris ova are fed to mice and certain other rodents, the larvæ, or a proportion of them, on being hatched out, bore their way into the liver and lungs. In the latter organs they appear in about a week's time, and, if the dose of eggs has been a large one, may cause fatal pneumonia. Stewart fed ascaris-infected mouse-dung to five pigs. Two of the experiments proved negative; in one pig a single female ascaris was found, and in the remaining two several sexually mature specimens. Stewart concludes that mice and rats serve the ascaris as intermediary host, man acquiring the infection in

Fig. 173.—Ovum of A. duodenale, x 250. (Photograph by Dr. J. Bell.)

food contaminated by the saliva of these rodents. It may be so, but further evidence is required before this conclusion can be accepted. Stewart's experiments certainly show that the larval ascaris leaves the lumen of the gut, and explain the purpose of the beak-like organ with which its cephalic end is furnished. Possibly when the larva quits the ripe egg in man it penetrates the gut and becomes encysted there or elsewhere for a time, undergoing developmental changes in anticipation of its return to the alimentary canal.

Symptoms.—In many instances the ascaris gives rise to no very noticeable symptom; in other instances it is to be credited with a number of ill-defined