’ 
i 
? 
LIFE HISTORY OF ASCARIS LUMBRICOIDES. 17 
the eggs that were fed to those animals. Obviously their infection 
could not reasonably be explained under the rat and mouse theory 
nor is it reasonable to suppose that they served as their own inter- 
mediate hosts and then swallowed their own feces containing larve 
that had been eliminated after passing through the lungs and intes- 
tine. The latter possibility, difficult to exclude in the case of pigs, 
is not one demanding serious consideration in the present instances, 
especially in view of the fact that the lamb and the kid were suck- 
lings only a few days old when used in the experiments. Further- 
more, the facts that sheep and goats are very unusual hosts of Ascaris 
and that in these experiments the parasites were found to be present 
in considerable numbers, in the case of the kid in enormous numbers, 
are facts of such a nature that the only possible conclusion is that 
the animals became infected from the eggs fed to them and that the 
larve after migrating from the intestine to the lungs and back agam 
into the intestine settled down in the intestine and continued their 
development toward maturity. It appears, therefore, justifiable to 
conclude, in the light of all the available evidence, that man and pig 
become infected with Ascaris as a result of swallowing the eggs of 
the parasite and not from swallowing larve that have already un- 
dergone partial.development in rats or mice. 
It is evident with respect to Ascaris suum and probably also Ascaris 
lumbricoides, in view of the probable identity of the pig Ascaris and 
the human Ascaris, that these parasites show different degrees of 
adaptation to different host animals. In some animals (rat, mouse, 
guinea pig, rabbit) they are able to pass through a portion of their 
development and reach a stage in which they are ready to settle down 
in the small intestine, but are not able to develop further and are 
eliminated in the feces; in other animals (sheep, goat) they can de- 
velop to a stage approaching maturity, and finally, in their usual 
hosts (pig, man), they are able not only to pass through the earlier 
stages of development which may occur in imperfectly adapted hosts, 
but to continue their growth to fertile maturity. : 
The growth of Ascaris larve after they have reached the intestine 
following their migration through the lungs appears to be rather 
slow. In experiments on human subjects Epstein (1892) found that 
Ascaris eggs appeared in the feces 86 days after feeding Ascaris 
lumbricoides eggs. As the feces were also examined 12 days prior 
to the examination at which the eggs were first found, the worms 
in these cases, therefore, began producing eggs from 74 to 86 days 
after infection, so that at least about 24 months were required for the 
full development of the parasites after infection. In our experiment 
with the lamb the worms were still considerably short.of their adult 
size 103 days after infection, but, no doubt, their growth had been re- 
137550°—20—Bull. 8173 
