16 BULLETIN 817, U. S. DEPT. OF AGRICULTURE. 
down in the intestines. Other possibilities are open. For example, 
if it is accepted that the larve migrate through the lungs of pigs as 
they do through the lungs of small experiment animals, which is 
shown to be the case by the result of the experiment with the second 
pig and other similar experiments, it is yet possible that the larve 
might be eliminated in the feces after migrating through the lungs, 
as in rats, Mice, guinea pigs, and rabbits, and later be picked up 
again by the pigs and only then become established in the small 
intestine. Thus the mode of infection would be similar to that as- 
sumed in the theory of the rat and mouse as intermediate hosts, 
except that the pigs would act as their own intermediate hosts, be- 
coming finally infected by larvee that had once passed through their 
own bodies instead of through the bodies of rats or mice. - : 
In our experiments there were also possible sources of infection 
other than the eggs that were fed. There may have been stray rats 
or mice about, or their feces may have been present in the feed, and 
in view of the small number of worms found in the intestine of the 
pig in the experiment recorded above, it may be supposed that they 
could have come from such a source. Owing to the difficulty of con- 
trolling experiments on pigs we have not yet succeeded in obtaining 
sufficient evidence from experiments on pigs alone to demonstrate 
conclusively that infection results from the ingestion of eggs and the 
subsequent migration of the larve through the lungs and back to the 
intestine, where they become established and develop to maturity. 
That such, however, is a fact we believe is proved by the following: 
The migration of the larve through the lungs occurs in animals of 
various species that ingest the eggs irrespective of whether such ani- 
mals are suitable hosts of the adult worms. This has been shown to 
be true of the rat, mouse, guinea pig, rabbit, goat, and pig. In man 
symptoms have been described by Mosler (in Leuckart, 1867) and 
Lutz (1888) that indicate the migration of Ascaris larve through the 
lungs. 
The Ascaris embryo prior to hatching from the egg is admirably 
fitted to withstand the hardships of existence outside the body of a 
host, and may remain alive for years, protected by the eggshell. The 
larva after reaching the stage in which it is eliminated from unsuit- 
able hosts, such as rats and mice, is poorly adapted to existence out- 
side the body of a host, is quickly killed by drying, and does not long 
survive even under favorable conditions. 
It has been shown by experiment that m lambs and Inds the worms 
will develop to a stage approaching maturity in the intestine after 
infection brought about by the ingestion of the eggs of the pig 
Ascaris (Experiments Nos. 22 and 23). 
In the experiments on the lamb and the kid there can be no. doubt 
that the partially developed worms found in the intestine came from 
