274 OTHER INTESTINAL ROUNDWORMS 



A scaris lumbricoides, is now usually looked upon as a variety 

 of the species which occurs in hogs in almost every country in 

 the world, and which is sometimes known as A. suilla. 



The life history of Ascaris is usually thought to be very 

 simple. The eggs, of which thousands are deposited by a single 

 female, develop within the eggshell outside of the human body, 

 in water, soil or manure piles, wherever the proper conditions of 

 temperature can be found. The eggs (Fig. 108) are about 0.06 mm. 

 long by 0.04 mm. wide (^fa by ^<y of an inch), elliptical in form 



with a thick transparent shell, 

 usually bile - stained, covered 

 over outside by irregular albu- 

 minous coats which give them 

 a rough warty appearance. 

 When passed from the diges- 

 B tive tract no sign of segmen- 

 Fi:: 108. Egg of Ascaris; A, surface tation can be seen. Under 



view showing warty albuminous coat; . 



B, same in "optical section," i.e., with favorable conditions OI Optl- 



microscope focused on center of egg in- mum temperature (33 C.), 

 stead of on surface. 



moisture, and oxygen supply 



the eggs develop in from 10 days to a month, but may be retarded 

 for a long time by low temperatures, excessive dryness or in- 

 sufficient oxygen. Although continued dryness at high tem- 

 perature may be fatal to the eggs, yet under natural conditions 

 in soil there is probably almost always sufficient moisture to 

 allow development to proceed, and the developed eggs may live 

 for several years. The eggs are extremely resistant to most 

 chemical reagents, even when used in very strong dilutions, so 

 that ordinary methods of disinfection would be of little value in 

 destroying them. The use of human faeces (night soil) as a fer- 

 tilizer undoubtedly results in wholesale contamination of vege- 

 tables and other garden products. 



The eggs only rarely hatch outside of the body; ordinarily 

 the larvae escape only after the eggs have been ingested and have 

 reached the small intestine. Since eggs injected under Lie skin 

 will hatch it is evident that some other factor besides the digestive 

 juices is influential in liberating the larvae. 



Stewart, in recent experiments in China, was the first to dem- 

 onstrate that A scaris might not develop directly in the intestine 

 of its host. Subsequent experiments by Stewart and also by 

 Ransom and Foster have fully corroborated Stewart's work. 



