178 A GENERA TION 



there is sometimes a stronger tendency to produce new organs at or 

 near the place of union than when unlike poles are united, but it 

 would be going too far, I think, to state that this is due to repulsion 

 of the parts, especially in the sense in which the like poles of a 

 magnet repel each other. It seems to be due rather to the two parts 

 failing to unite into a whole organization, each retaining the same 

 structural basis that it had before grafting, but this is a very different 

 principle from that of an attraction and repulsion of the parts, and 

 the question of the union of the parts appears also to be a different 

 question from that of the organization of the parts themselves. 



In the mammals, and in general in all forms in which there is a 

 dependence of the parts on each other, it is impossible to carry out 

 grafting-experiments on the same scale as those described in the pre- 

 ceding pages. The principal difficulties are to make the parts unite, 

 and to supply nourishment and oxygen to the graft. Owing to the 

 dependence of the parts of the body on each other for a constant sup- 

 ply of oxygen and food derived from the blood, as well as for the 

 removal of the waste products, the parts cannot remain alive, or even 

 in good condition, while new connections are being established. For 

 this reason, as well as for others, it would not be possible, for instance, 

 to graft the arm of a man upon another man. The tissue may have the 

 power of uniting even in this case, as is seen when the bone is broken 

 and subsequently reunited, but the difficulty would be in supplying the 

 grafted arm with nourishment, etc., during the long time required for 

 the union to take place. Smaller parts of the body may be success- 

 fully grafted, and there are several recorded cases in which parts of a 

 finger, or of the nose, are said to have been cut off and to have reunited 

 after being quickly put back in place. Pieces of human skin may be 

 grafted without great difficulty upon an exposed surface, and it has 

 been -said that small pieces succeed better than larger ones, owing, 

 most probably, to their being able to absorb sufficient oxygen, etc., and 

 keep alive until new blood vessels have grown into the grafted piece. 



There are a number of old and curious observations in regard to 

 cases of grafting in higher animals. It was found by Hunter and by 

 Duhamel that the spur of a young cock could be grafted upon the 

 comb, when it continued to grow to its normal size. The comb, being 

 richly supplied with blood, furnished the nourishment for the growth 

 of the spur. Fischer transplanted the leg of an embryo bird to the 

 comb of a cock, or of a hen, where it grew at first, but after some 

 months degenerated. Zahn transplanted the fcetal femur to the kid- 

 ney, where it grew for a time, but later degenerated. Bert transplanted 

 the tail of a white rat to the body of Mns decnmanns, where it continued 

 alive ; but he found that the tail of the field mouse, Mns sylvaticus, 

 did not grow so well on the rat, and the tail of a rat would not unite 



