Recent investigations by Stewart and others have demonstrated that when eggs of Ascaris lumbricoides containing fully formed vermiform embryos are swallowed, they hatch in the intestine and release the embryos which do not immediately settle down, but leave the intestine and pass to the liver, then to the lungs and finally back to the intestine by way of the trachea, esophagus, and stomach. During this journey, which commonly requires about ten days, the larvae not only attain a size much larger than that of the newly hatched worms, but undergo developmental changes. This migration and development can be followed in laboratory animals (guinea-pigs, rabbits, rats and mice), there being no apparent essential difference from that occurring in the pig (experimentally verified in various respects) and presumably (by indirect evidence) in man, except that in the first instance the larvae are unable to continue their development to maturity after returning to the intestine and soon pass out of the body in the feces to perish.