10 BULLETIN 817, U. S. DEPT. OF AGRICULTURE. 



Although we are ignorant of the determining cause of hatching, 

 nevertheless we can reach certain conclusions as to the place and 

 manner in which hatching occurs, as follows: When Ascaris eggs 

 containing fully developed embryos are swallowed they do not regu- 

 larly hatch in the stomach, but pass to the small intestine, where they 

 begin to hatch within a few hours after the eggs are swallowed. The 

 eggshell is not dissolved by the digestive juices, the embryo being re- 

 leased by a split in the shell through which it emerges by its own 

 efforts. Ascaris eggs maj^ hatch not only in animals in which the 

 parasites can develop to maturity, but apparently in almost any mam- 

 mal that swallows the eggs, provided the embryos within the eggs 

 have reached a stage in which they are ready to hatch. 



LARVAL STAGES OF ASCARIS. 

 MIGRATIONS OF LARV^ IN BODY OF THE HOST. 



It has been stated that in artificially infected rats, mice, and other 

 animals newly hatched Ascaris larvae may be recovered in the feces 

 within a few hours after the eggs have been swallowed. Stewart 

 (1916-1918), however, discovered that not all the larvae were thus 

 eliminated, but that some migrated out of the alimentary tract into 

 other parts of the body and soon appeared in the liver. Stewart 

 (1916a) found larvae in the liver of a mouse (in dilated blood capil- 

 laries) which died four days after being fed eggs of A. lumhricoides. 

 In this animal the larvae had also reached the lungs (in the air vesi- 

 cles). He found larvae of Ascaris ina/r'ginata in the liver of mice 

 within 24 hours after administration of the eggs, none having yet 

 appeared in the lungs (Stewart, 1918a, p. 194). Our experiments 

 show that the larvae after leaving the lumen of the intestine soon 

 appear in the liver, and may be found there before they are evident 

 in the lungs. For example, a mouse killed 51 hours after feeding 

 with Ascaris suum eggs showed numerous larvae in the liver, but 

 none could be found in the lungs or spleen. Frequently as late as 

 five days after feeding Ascaris eggs to mice we found larvae only in 

 the liver, none having yet reached the lungs. In one instance, eight 

 days after it had been fed Ascaris sv/u/m eggs, a mouse was killed and 

 larvae were found only in the liver. 



The newly hatched larvae of Ascaris suum measure about 0.2 

 to 0.3 mm. in length. In the liver the larvae may grow to a length 

 of 0.86 mm. (mouse nine days after infection, lungs and various 

 other organs also infested), reaching a stage of development similar 

 to that of larvae of corresponding size in the lungs. 



Ascaris larvae commonly disappear from or at least become scarce 

 in the liver within from 4 to 10 days after infection, but we have 

 found numerous larvae in the liver of a mouse as late as 23 days after 



