172 THE BIOLOGICAL BASIS OF INDIVIDUALITY 



the second partner, and then an ingrowth of a peripheral nerve into the second 

 animal may take place and cross-reflexes between the two partners can be 

 established. Apart from these data, the sequence of events taking place in the 

 tissues adjoining the area of union has not yet been sufficiently examined 

 microscopically to determine whether at this point accumulations of lympho- 

 cytes occur comparable to those which are found around and in the ordinary 

 transplants of homoiogenous tissues. However, it is conceivable that the 

 constant supply of its own bodyfluids to the tissue of each partner alters the 

 reaction of the lymphocytes to the strange tissue of the parabiotic partner, 

 which would otherwise be found. 



The general changes occurring at the site of union may vary greatly in dif- 

 ferent cases. In some instances, the tissues of the two partners may not unite 

 well and may separate soon after the operation ; or a temporary union may oc- 

 cur, which is followed before very long by a separation ; again, there may be 

 a long-continued union, which still may ultimately be succeeded by a shrink- 

 ing of the skin flaps and a separation. In the majority of experiments the 

 union seems to last not longer than a few weeks, but Morpurgo succeeded in 

 keeping a pair of parabiotic rats united for 9-12 months, and one case is 

 known in which the union between two rats continued for as long as two 

 years and five months (Goto). Rats are apparently best suited, and rabbits 

 next best, for parabiotic union, and it seems that union between other 

 species, such as cats or dogs, and perhaps even mice, can be established only 

 with greater difficulty. The results of parabiosis are most favorable between 

 young litter mates of about the same weight and sex; however, a union 

 between rats not belonging to the same litter nor to the same sex also may 

 succeed, but for technical reasons it is advisable to choose partners of about 

 the same age. As stated, the establishment of a syngenesio-parabiosis, where 

 the partners are brothers or sisters, is most favorable, although a homoi- 

 ogenous parabiosis may also succeed, but on the average less readily. Still 

 more difficult are unions between different races; but Irwin joined success- 

 fully different races of doves. A parabiotic union between individuals be- 

 longing to different species (heterogenous parabiosis) may, in rare instances, 

 succeed for a very short time; thus Lambert joined a rat and a mouse for a 

 maximum period of eight days, and even a union between a guinea pig and 

 rat succeeded for as long as eight days. However, under these conditions 

 no real wound healing occurred at the point of union and afterwards the 

 skin flaps separated as in cases of disharmonious homoiogenous union. 



In parabiosis, a small amount of blood, of peritoneal fluid, of lymph and 

 interstitial fluid constantly passes from one partner into the other and here 

 it is mixed with a much larger quantity of the animal's own fluids. There is 

 reason, therefore, for assuming that the fluids of one animal reach, the cells 

 of the strange organism, but only in very great dilution, and before they come 

 into actual contact with the cells of the partner, they meet a current of fluid 

 from the latter passing in the opposite direction, namely, from the cells to- 

 wards the capillaries. The tissues thus constantly create their own indi- 

 viduality differentials, which move towards and mix with the very dilute 

 strange differentials in such a way that a gradient of these differentials de- 



