LIFE HISTORY OF ASCARIS 275 



in the intestine to develop further, though dead ones are found 



in the fseces. Only about one per cent ever reach the lungs. From 



the sixth to the tenth days the larvae pass from the bloodvessels 



into the air sacs and bronchial tubes of the lungs and thence 



through the trachea to the 



mouth. If the pneumonia 



does not prove fatal the host 



recovers in 11 or 12 days and 



by the sixteenth day is free 



from parasites. The largest 



larva observed (Fig. 109 A) 



was found in the lung of a rat 



on the tenth day after infec- 



tion; it measured 1.32 mm. . 



riG. 109. Developmental stages of 

 (about -fa of an inch) in length. Ascaris; , freshly hatched larva; b, larva 



fpn from lun & of rat on tenth da Y after infec- 



tap tion x 13Q> (Adapted from Stewart . ) 

 water but can survive 24 hours 



on damp bread and two days in a rat V lung. Capt. Stewart be- 

 lieves that these experiments suggest that man is infected by 

 food contaminated by larvae which have emigrated actively from 

 the mouth of a rat while the rat was nibbling. Hogs were suc- 

 cessfully infected by larval worms from the lungs of rats. 



That this is the usual life history of Ascaris must certainly be 

 doubted. When it is remembered that a large per cent of chil- 

 dren in all tropical countries are infected, and often very heavily 

 infected, the improbability of more than a few at most of the 

 infections arising in the manner indicated above is apparent. 

 Of 5000 eggs ingested by a rat. not more than 50 were recovered 

 by Stewart from the lungs, and of these only very few could 

 possibly be successful in reaching a human intestine by way of 

 moist foods nibbled by rats within the preceding 24 hours. 

 Moreover several very eminent parasitologists have been success- 

 ful in producing infection by feeding ripe eggs to hogs and also to 

 man. It seems much more probable that Capt. Stewart's experi- 

 ments may be interpreted as suggesting that Ascaris normally 

 migrates through the tissues, as do the hookworms, before becom- 

 ing settled in the intestine, and that the worms recovered from 

 the lungs and mouth are on their way back to the digestive tract 

 of the rat. That they do not become established there may be 

 explained by their inability to live in the intestine of this host.* 



* Since this book has gone to press Ransom and Foster have published 

 results of experiments which verify this conclusion, and which show that only 

 very young animals are readily susceptible to infection. 



