44 BULLETIN 817, U. S. DEPT. OF AGRICULTURE. 
finally pass out of the body in the feces. They may be found in 
the liver as early as 2 days after infection, in the lungs and 
trachea as early as 3 days after infection, and in the alimentary 
tract after their passage through the lungs as early as 6 days after 
infection. They are common in the lungs a week to 10 days after 
infection, becoming scarce in the liver as they become numerous in 
the lungs. Within a little over two weeks after infection all or 
practically all the larvee are usually eliminated, but have been found 
still present and active in the liver, lungs, and alimentary tract as 
late as 23 days after infection. 
In young goats and lambs the larvze of Ascaris suwm after migrat- 
ing through the lungs settle down in the small intestine and undergo 
development approaching maturity, these animals thus being better 
adapted as hosts than rats, mice, guinea pigs, and rabbits. 
In pigs Ascaris larve after migrating through the lungs settle 
down in the small intestines and develop to maturity, and presum- 
ably the same cycle occurs in human beings. 
Rats and mice play no part in the normal life history of Ascaris. 
The behavior of the larve in these animals and in guinea pigs and 
rabbits may be considered simply an expression of imperfect adapta- 
tion of the parasites to existence in these hosts. 
In pigs and human beings Ascaris may develop to maturity within 
two and one-half months after infection. 
In guinea pigs the larve apparently reach a larger average size 
than in mice in the same length of time, and still larger in rabbits. 
Migrating Ascaris larve produce destructive lesions in the liver 
and lungs, especially in the latter. 
Some larvee die during their migrations. Dead and degenerated 
larvee have been found in the lungs as late as 86 days after infection, 
and in the liver 296 days after infection. 
The invasion of the lungs by Ascaris larve may result in a serious 
and sometimes fatal pneumonia, which appears in a week to 10 days 
after ingestion of the eggs 
Young pigs are more ee pulie to Ascaris infection than older 
animals, but may not entirely lose their susceptibility with advancing 
age. 
No evidence has been obtained that one infection with Ascaris ren- 
ders animals less susceptible to later infections. 
If properly incubated Ascaris eggs are injected beneath the skin 
they will hatch, and the larve will migrate to the lungs, where they 
may be found a few days after cenen of the eggs to have reached 
the same stage of development as they would if infection had oc- 
curred from swallowing the eggs. 
The larve of Belascaris marginata undergo migrations in rats simi-. 
lar to those of Ascaris suum and A. lumbricoides. 
