GROWTH AND REGENERATION 273 



not only have pieces of skin or bone of one animal replaced 

 corresponding parts of another, but whole sections of arteries, 

 and even kidneys and other organs, have been transplanted 

 from one individual to another. 



With the improvement in the technique of such operations it is 

 not unthinkable that we may be able in time to replace a person's 

 diseased kidney, for example, with the kidney of a dog or a sheep, if 

 that is found suitable, or with the kidney of another person who has 

 recently died or who has had to have his kidney removed for some 

 reason. This principle is now applied in the repairing of crushed 

 bones ; the injured part is neatly cut away, and the missing portion 

 is replaced with a suitable piece of the same size taken from the leg 

 of a sheep. The chief obstacle to the practical use of grafting with 

 human beings and other warm-blooded animals lies in the fact that 

 the blood, through the chemical activity of the white corpuscles, 

 develops substances that are antagonistic to foreign proteins (see 

 Chapter XXXVII). 



