H 4 6 REPRODUCTION 



been transplanted in this way from one dog to another. Although 

 from the operative point of view successful, the homoeo-transplants 

 do not permanently survive. Reimplantation of organs, however, 

 in one and the same animal with suturing of the bloodvessels has 

 often been successfully performed. Such organs as the kidney live 

 and function after reimplantation, and the operation is now a 

 recognized physiological method of insuring that an organ has been 

 completely disconnected from the central nervous system, since even 

 the nerves running in with the bloodvessels must have been cut. 

 It has been shown that a reimplanted kidney suffices to maintain a 

 dog in complete health for an indefinite period after the removal 

 of the other kidney. 



In the case of structures like the large bloodve c sels, which 

 perform mainly a passive mechanical function, homoeo-transplanta- 

 tion succeeds. Segments of arteries preserved in cold storage 

 for a few days or even weeks, and even portions of arteries fixed by 

 formaldehyde, have been transplanted so as to take the place of 

 segments removed from arteries of living animals, and have con- 

 tinued to function perfectly for long periods. Portions of veins 

 have also been used to fill up gaps in arteries. Even heteroplastic 

 vascular grafts have been found to succeed, portions of dog's 

 arteries, e.g., grafted into a cat, and portions of rabbit's, cat's, or 

 human arteries grafted into a dog. Doubtless the favourable result 

 is largely due to the fact that the mechanical function of the large 

 arteries can be discharged even by a dead tube of the requisite 

 strength, and with the smooth interior presented by a dead endo- 

 thelial lining (Carrel, Guthrie). 



Parabiosis. Not only may an organ or a portion of tissue from 

 one individual be engrafted on another, but two individuals may be 

 so united that a greater or smaller degree of physiological in- 

 timacy is produced between them. Occasionally, as in the famous 

 Siamese twins, an anomaly of development results in such close 

 anatomical union of the circulatory and other systems that in 

 certain respects the two individuals constitute almost a single 

 organism, and cannot be separated by surgical interference. A 

 less intimate union can be established experimentally by opening 

 the peritoneal cavities of the two animals, and suturing the skin 

 and connective tissue together so as to permit of permanent 

 communication. Pairs of animals living in this condition (so- 

 called parabiosis) have been utilized for the study of certain 

 questions in immunity. White rats have been kept alive in para- 

 biosis for as long as thirty-four days in order to test the question 

 whether destructive antibodies for cancer are present in the circula- 

 tion (Rous), since it has been shown that circulating antibodies easily 

 pass from one to the other of such a pair of animals (Ehrlich). 



