LIFE HISTORY OF ASCARIS LUMBRICOIDES. 17 



tne eggs that were fed to those animals. Obviously their infectioii 

 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 larvae 

 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 

 larvae after migrating from the intestine to the lungs and back again 

 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 larvse that have already un- 

 dergone partial development in rats or mice. 



It is evident with respect to Asearis suwm and probably also Ascaris 

 lumhricokles^ in view of the probable identity of the pig Ascaris and 

 the human Ascans, 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 readj^ 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 larvae 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 

 T/umibricoides 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 2^ 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, 817 3 



