NEMATHELMINTHES. ROUND-WORMS 35 



life cycle of the worm cannot take place in the single person, so that 

 the number of worms in the intestine can be increased only by a new 

 infection. 



The female worm produces the eggs, which are about 35X70 micra 

 (Kooo mm.) in size, in enormous numbers and they pass to the ex- 

 terior with the feces, as many as 4,000,000 it is estimated, with one stool. 

 Desiccation, direct sunlight, too much water and, generally, freezing 

 will kill these eggs, which have usually begun to segment when passed 

 out with the stool. 



The rapidity of development depends upon the temperature and 

 moisture, but the larvae become infective in four or five days; under 

 favorable conditions these larvae may live for months. 



Infection may take place either by the mouth or, more commonly, 

 through the skin. It is the entrance of the tiny larvae through the skin, 

 more often of the feet, but of any part of the body, that causes the ir- 

 ritation known as " ground itch," "foot itch," "dew itch," etc. A 

 large percentage of hookworm disease histories show cases of ground 

 itch. The mode of entry through the skin was discovered in 1898 

 by Looss in a very interesting way. While experimenting with hook- 

 worm larvae he spilled some of them on his hand; he noticed a burning 

 sensation and in a few minutes the larvae had disappeared. After the 

 proper interval, about two months, he found himself suffering with 

 the hookworm disease. To determine what had become of the larvae 

 that were spilled on his hand he poured some larvae on the leg that 

 was about to be amputated from a boy; on sectioning the skin of 

 this leg, after amputation, he found the larvae had worked their way 

 through the skin by way of the hair follicles, sweat ducts, etc. To 

 follow the further course of the larvae he placed some of them on 

 the skin of a number of dogs which were killed and examined at 

 various intervals. In this way he worked out the entire course of the 

 larvae from the skin to their final resting place in the intestine, which 

 is"as follows: after passing through the skin they enter the subcu- 

 taneous blood vessels and are carried by the blood current to the heart; 

 thence to the lung capillaries, from which they burrow through the 

 tissues into the lung alveoli, thence they work their way up the trachea 

 to'the oesophagus, down which they pass to the stomach and intes- 

 tine. It is the tiny lesions made by the larvae in passing through 



