44 Chemical Basis of Genus and Species 



/. The Incompatibility oj Species not closely Related 



2. It is practically impossible to transplant organs 

 or tissues from one species of higher animals to an- 

 other, unless the two species are very closely related; 

 and even then the transplantation is uncertain and 

 the graft may either fall off again or be destroyed. 

 This specificity of tissues goes so far that surgeons 

 prefer, when a transplantation of skin in the human is 

 intended, to use skin of the patient or of close blood 

 relations. The reason why the tissues of a foreign 

 species in warm-blooded animals cannot grow well on 

 a given host has been explained by the remarkable 

 experiments of James B. Murphy of the Rockefeller 

 Institute.' Murphy discovered that it is possible to 

 transplant successfully any kind of foreign tissue upon 

 the early embryo of the chick. Even human tissue 

 transplanted upon the chick embryo will grow rapidly. 

 This shows that at this early stage the chick embryo 

 does not yet react against foreign tissue. This lack 

 of reaction lasts until about the twenty-first day in 

 the life of the embryo ; then the growth of the graft not 

 only ceases but the graft itself falls off or is destroyed. 

 Murphy noticed that this critical period coincides with 

 the development of the spleen and of lymphatic tissue 

 in the chick and that a certain type of migrating cells, 



» Murphy, J. B., Jour. Exper. Med., 1913, xvli., 482; 1914, xix., 181; 

 xix., 513; Murphy and Morton, J. J., Jour. Exper. Med., 1915, xxii., 204. 



