2 BULLETIN 817, U. S. DEPT. OF AGKICULTUEE. 



important discovery that if a rat or a mouse is fed Ascans eggs many 

 of the larvae which escape from the eggs in the intestine wander out of 

 the alimentary tract, and, apparentlj'^ aided by the circulation, reach 

 the liver and later the lungs, meanwhile undergoing considerable 

 growth and development. 



Many years ago Davaine (1863) observed newly hatched larvte 

 in the feces of rats a few hours after feeding them Ascans eggs, but 

 Stewart was the first to discover that in these animals not all the 

 young worms that hatched were promptly eliminated but that some 

 penetrated the wall of the alimentary tract and reached other loca- 

 tions in the body, meanwhile increasing in size and undergoing cer- 

 tain developmental changes. He determined further that the larvae 

 in the lungs do not remain there, but migrate up the trachea and can 

 be found in the mouth. On the basis of this fact Stewart (1916b) 

 in one of his papers suggested that rats and mice that liad become 

 infected by swallowing the eggs of Ascai'^s might later transfer the 

 parasites to human beings or pigs by contaminating food with 

 saliva containing the young worms. Stewart, however, in continuing 

 his investigations found that after the larvae passed up the trachea 

 they then passed down the esophagus, reached the intestine, and 

 tended to accmnulate in the cecum. Finally they passed out of the 

 intestine in the feces without undergoing any material change in 

 size or structure from' that presented by the stage attained in the 

 lungs. 



Certain experiments with pigs had failed to result in definite proof 

 that these animals became infected with Ascaris through swallowing 

 the eggs, and Stewart therefore offered the hj^pothesis that rats and 

 mice act as intermediate hosts in the life cycle. According to this 

 hypothesis AscaHs eggs contained in the feces of infested human 

 beings or pigs after reaching the infective stage are swallowed by 

 rats or mice. In these hosts the young worms hatching from the eggs 

 migrate from the intestine to the lungs and back again to the intes- 

 tine and reach a stage of development considerably in advance of 

 that characterizing the embryo within the egg. They then pass out 

 of the body of the rat or mouse in the feces. If food contaminated 

 with the feces of infested rats or mice is eaten by human beings or 

 pigs the worms thus reach their final host, in whose intestine they 

 settle down and develop to maturity. The completion of the life 

 cycle thus requires, according to Stewart's explanation, an alternation 

 between two hosts— the final host, man or pig, and the intermediate 

 host, rat or mouse. 



Lane (191T) and Low (1918) have given a number of reasons for 

 doubting the validity of Stewart's explanation of the life cycle of 

 Ascans, and the present writers in a preliminary note (Ransom and 



