BLOOD VESSEL ANASTOMOSIS 171 



But it is doubtful whether such an occurrence would take place in rodents, 

 in which experiments of this kind are carried out most frequently; here, 

 apparently, the actions of the individuality differentials greatly predominate 

 over blood group antigens, if the latter play any significant role at all in 

 these animals. The same considerations would apply also as far as the 

 interaction of the partners in typical parabiosis is concerned; here, too, it 

 is very doubtful whether in experiments in organisms other than man and 

 monkeys, blood group antigens would be of any importance. But in the 

 latter, they may be effective, and in man also the stage of pregnancy may 

 be of importance. In women it has been observed that especially the antigen 

 Rh may pass from the fetus to the mother, causing the production of anti- 

 bodies which then may lead to changes in the fetus (Wiener, P. Levine). 



Parabiosis and individuality differentials. Parabiosis is an extension of the 

 method of transplantation by pedicle flaps, in which, in addition, union 

 usually takes place between two individual organisms by the joining together 

 of small areas of the peritoneal wall, and, in some cases, also of the intestines. 

 However, it is a method devised primarily for the purpose of joining to- 

 gether or transplanting on each other, two organisms which are able to live 

 independently, and which, in certain respects, continue to live independently 

 even after the union has been accomplished. Each organism takes its own 

 food and maintains its own metabolism ; each is united with its partner 

 mainly by means of capillary anastomoses, which gradually increase up to 

 about two weeks after operation, after which time there may be again a 

 diminution in the number of anastomosing capillaries owing to the pressure 

 exerted by the developing scar at the site of junction. It has been found that 

 the connection by lymph vessels is richer than that by blood capillaries ; 

 occasionally, though, there may be an additional connection by large vessels 

 in the omentum. The partners, corresponding to host and transplant, continue 

 therefore to be perfused largely by their own blood supply; but, at the 

 same time a not very intense, but continuous, inflow of strange body fluids 

 occurs, thus transferring to one partner products of the intermediary metab- 

 olism from the other partner, and above all, transferring also substances 

 carrying the individuality differentials of the other partner. 



Locally, at the point of union, at first a large amount of granulation 

 tissue develops, consisting largely of fiibroblasts but containing also various 

 kinds of leucocytes; it is uncertain whether this abundant tissue formation 

 represents merely the sum of that which would normally be furnished by 

 each partner in the course of wound healing, or whether in addition the in- 

 fluence of substances strange to each partner exerts a special stimulation on 

 the granulation tissue. In contrast to the blood vessels of the two organisms 

 which communicate with each other and may grow from one animal into the 

 territory of the other, no spontaneous ingrowth of peripheral nerves takes 

 place, the nerves of each organism remaining separate. This is probably the 

 reason why diseases like rabies or tetanus, which are propagated mainly by 

 way of peripheral nerves, do not in parabiosis progress spontaneously from 

 one partner to the other. However, as Morpurgo has shown, it is possible to 

 establish experimentally a union of a nerve of one partner with a nerve of 



