274 OTHER INTESTINAL ROUNDWORMS 



Ascaris 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 (:i^^ by ^h^ of an inch), eUiptical in form 



with a thick transparent shell, 

 usually bUe - stained, covered 

 over outside by irregular albu- 

 minous coats which give them 

 a rough warty appearance. 

 When passed from the diges- 

 D tive tract no sign of segmen- 

 Fic. 108. Egg of Ascaris; A, surface ^^^^^^^ ^^^ ^e Seen. After a 



view showing warty albuminous coat; 



B, same in "optical section," i.e., with month or six weeks Under fa vor- 



microscope focused on center of egg in- ^^ile conditions in Soil Or water 

 stead of on surface. 



the embryo will have devel- 

 oped, and can then be seen rolled up within the shell. Even 

 eggs which have been dried and exposed to the sun for months 

 may contain active embryos. The egg may remain for months 

 or years in this condition, resistant to both dr5dng and freezing, 

 until swallowed by a human being or other susceptible animal. 

 In the dry condition the eggs may be blown about by the wind 

 or carried on the feet of flies. The use of hmnan faeces (night- 

 soil) as a fertilizer undoubtedly results in wholesale contamina- 

 tion of vegetables and other garden products. 



When swallowed by a suitable host the hard shell of the egg is 

 dissolved off and the parasite is liberated in the small intestine. 

 After about five or six weeks sexual maturity is reached, and the 

 production of eggs begins again. 



Recent experiments by Capt. Stewart in Hong Kong indicate 

 that at least under some conditions Ascans may go through an- 

 other phase of development in its life history. Ripe eggs in- 

 gested by rats hatch in the intestine, and the larva (Fig. 109B) 

 invade the tissues of the rats. In from fom- to six days some of the 

 larvse are found in the bloodvessels of the lungs, hver and spleen, 

 giving rise to symptoms of pneumonia. None of them remain 



