398 
Journal of Agricultural Research 
Vol. XI, No. 8 
of a more extended statement in a future paper, may be briefly given 
as follows: 
The development of A. lumbricoides and closely related forms is direct, 
and no intermediate host is required. 
The eggs, when swallowed, hatch out in the alimentary tract; the em¬ 
bryos, however, do not at once settle down in the intestine, but migrate 
to various other organs, including the liver, spleen, and lungs. 
Within a week, in the case of the pig Ascaris, the migrating larvae may 
be found in the lungs and have meanwhile undergone considerable 
development and growth. 
From the lungs the larvae migrate up the trachea and into the esophagus 
by way of the pharynx, and this migration up the trachea may already 
become established in pigs, as well as in artificially infected rats and mice, 
as early as a week after infection. 
Upon reaching the alimentary tract a second time after their passage 
through the lungs, the larvae, if in a suitable host, presumably settle 
down in the intestine and complete their development to maturity; if in 
an unsuitable host, such as rats and mice, they soon pass out of the body 
in the feces. 
Heavy invasions of the lungs by the larvae of Ascaris produce a serious 
pneumonia which is frequently fatal in rats and mice and apparently 
caused the death of a young pig one week after it had been fed with 
numerous Ascaris eggs. 
It is not improbable that ascarids are frequently responsible for lung 
troubles in children, pigs, and other young animals. The fact that the 
larvae invade the lungs as well as other organs beyond the alimentary 
tract and can cause a serious or even fatal pneumonia indicates that these 
parasites are endowed with greater capacity for harm than has hereto¬ 
fore been supposed. 
Age is a highly important factor in determining susceptibility to infec¬ 
tion with Ascaris, and susceptibility to infection greatly decreases as the 
host animal becomes older. This, of course, is in harmony with the 
well-known fact that it is particularly children and young pigs among 
which infestation with Ascaris is common, and that Ascaris is relatively 
of rare occurrence in adult human beings and in old hogs. 
