BLOOD VESSEL ANASTOMOSIS 173 



velops. Hence the action of the bodyfluids of one partner on the other 

 is very imperfect. We notice, accordingly, that substances which have a 

 relatively low molecular weight and are not colloidal, such as KI, also prod- 

 ucts of the intermediary metabolism and certain toxins pass readily from one 

 partner to the other. Other substances which have presumably larger mole- 

 cules, with more colloidal properties, or substances such as some hormones 

 which have strong affinities for organs in their own body and are here held 

 back more readily, pass with greater difficulty and only under favorable 

 conditions from one partner to the other. For instance, following extirpa- 

 tion of the kidneys of one partner, it is this partner which is primarily af- 

 fected by the lack of these excretory organs ; it suffers therefore from 

 edema, also, in some cases, from hypertrophy of the left ventricle of the 

 heart, and only under favorable circumstances can the compensatory hyper- 

 trophy which may occur in this animal save it from death by uremia 

 (Morpurgo). 



Similarly the interaction between the sex hormones takes place only im- 

 perfectly between the two partners, but castration of one partner may exert 

 a stimulating effect on the sex organs of the other partner through the 

 intermediation of the hypophysis (P. E. Smith, Matsuyama. Kellars, Mar- 

 tins, and others). If a pregnant and a pon-pregnant rat are united, a growth 

 may set in also in the mammary gland of the non-pregnant partner, but 

 the reaction in the latter is weaker and delayed, indicating that some inhibi- 

 tion exists in the transmission of this effect. A passive immunity can be readily 

 transferred from one partner to the other, but an active immunity can be 

 induced in one animal through injections of the antigen into his partner only 

 with greater difficulty; larger quantities of antigen must be used for this 

 purpose. Likewise, if an animal susceptible to leukemia is united by means 

 of parabiosis with a partner not susceptible to this disease, both individuals 

 retain their specific degree of resistance to it, the leukemia being transmitted 

 only to the susceptible individual ; but transmission is more readily effected 

 by injecting the leukemic blood cells into the susceptible animal directly 

 than by injecting them into the non-susceptible partner. Also, in the case of 

 a difference in susceptibility to transplanted tumors between two partners, 

 as a rule each partner retains its specific state of susceptibility or lack of 

 susceptibility; only in the case of Jensen rat sarcoma Zakrzewski observed 

 that a Wistar rat, not susceptible to this tumor, can be made susceptible to 

 it by the parabiotic union with a susceptible Warsaw rat. According to 

 Simonnet and Pretresco, if a normally faster growing male rat and a more 

 slowly growing female rat are joined in parabiosis, the growth rate of the 

 partners is intermediate between the normal developmental rates of the two 

 partners; an effect is thus transmitted in this case from each partner to the 

 other. 



As to the general effect of two parabiotic partners on each other, it is 

 possible, in many instances, to distinguish two successive phases. As a rule 

 there is at first a harmonious phase, in which both animals are relatively 

 strong ; this is followed sooner or later by a disharmonious phase, in which one 

 of them becomes weak and atrophic, and eventually may die; but also the 



