2 BULLETIN 817, U. S. DEPT. OF AGRICULTURE. 
important discovery that if a rat or a mouse is fed Ascaris eggs many 
of the larve which escape from the eggs in the intestine wander out of 
the alimentary tract, and, apparently 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 larvee 
in the feces of rats a few hours after feeding them Ascaris 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 larve 
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 had become 
infected by swallowing the eggs of Ascaris 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 larve passed up the trachea 
they then passed down the esophagus, reached the intestine, and 
tended to accumulate 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 hypothesis that rats and 
mice act as intermediate hosts in the life cycle. According to this 
hypothesis Ascaris 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 (1917) and Low (1918) have given a number of reasons for 
doubting the validity of Stewart’s explanation of the life cycle of 
Ascaris, and the present writers in a preliminary note (Ransom and 
