16 BULLETIN 817, U. S. DEPT. OF AGRICULTURE. 



down in the intestines. Other possibilities are open. For example, 

 if it is accepted that the larvae 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 larvae 

 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 jfinally infected by larvae 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 larvae 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 larvae 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 larvae through the 

 lungs. 



The Asoaris embryo prior to hatching from the Qgg is admirably 

 fitted to withstand the hardships of existence outside the bod}' of a 

 host, and may remain alive for years, protected by the eggshell. Tlie 

 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 in lambs and kids 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 

 Asoaris (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 



