152 ANIMAL LIFE AND HUMAN PROGRESS 



morphosis takes place in moist soil. Many years ago Leuckart 

 traced this extra-corporeal phase in a closely aUied species 

 which infests dogs, and found that the infective stage is taken 

 into the body in drinking-water. 



In 1894 Looss, while studying the metamorphosis of the 

 worm as it occurs in man in Egypt, accidentally let a drop of 

 water, containing the developing worms, fall upon the back 

 of his hand. Some minutes later an intense itching sensation 

 developed at the same spot. Seeking an explanation for this 

 annoyance he concluded that it must have been due to some- 

 thing in the water which had been wiped off — either to some 

 irritant or toxic substance secreted by the growing worms or 

 to invasion of the skin by these microscopic forms. A part 

 of the same culture was then filtered and the clear water was 

 apphed to another portion of the skin. No irritation followed. 

 The young worms, in fresh water, were then applied and again 

 intense irritation was set up. 



From these experiments Looss concluded that the ankylo- 

 stome worm is able to enter the human body through the skin. 

 Some weeks later additional evidence was forthcoming, for 

 symptoms of hookworm infection began to manifest themselves 

 and the eggs of the hookworm were passed in numbers in the 

 excrement. 



With metamorphosed larvae of an allied species, Ankylo- 

 stoma caninum, a natural parasite of the dog, Looss then 

 carried out an exhaustive series of experiments which con- 

 clusively proved that, after piercing the skin, the larvae enter 

 the small veins and are carried to the heart. Thence they 

 pass in the blood-stream to the lungs. In the lungs they are 

 eventually stopped in the fine capillaries, through which they 

 are too large to pass. The larvae then burst into the air-sacs, 

 whence they are carried by the mucus into the bronchioles. 

 They are found later in the trachea, and, being swallowed, 

 pass into the gut, where they finally attain their full maturity. 



This entirely novel route was at first received with 

 scepticism but was quickly confirmed by other workers. In 

 England, and in America, volunteers submitted to experi- 

 mental infection. Successful and indeed severe infections 



