MODIFICATION OF REACTION OF THE HOST 179 



tissue, and it is possible that at a later period a moderate newformation of 

 acini may have taken place in the graft in this site. We may then conclude 

 that while in and around transplants of normal homoiogenous tissue the 

 lymphocytic infiltration is diminished in the brain, the connective-tissue re- 

 action is at least as marked as after subcutaneous transplantation. The mech- 

 anism which exerts a certain protective influence on homoiogenous grafts in 

 the brain is therefore exactly the reverse of that which is active in young, as 

 compared to older hosts. In young hosts the connective-tissue reaction is 

 diminished, whereas the lymphocytic infiltration may be very marked; in the 

 brain, the connective-tissue reaction may be quite pronounced, whereas the 

 lymphocytic infiltration is weak. As to the difference in the mode of reaction 

 in brain and in subcutaneous tissue of young animals, it seems improbable that 

 this is caused by a lack of individuality differentials in the brain ; it is more 

 probable that the blood-brain barrier (L. Stern) prevents the homoio-toxin 

 from reaching the brain in full strength, or at least diminishes its effective- 

 ness. However, the possibility cannot be excluded that also other factors may 

 be involved in this process. 



Likewise in the anterior chamber of the eye transplantations, especially of 

 organs with internal secretion, have been shown to succeed better than those 

 made subcutaneously or intraperitoneally. Of considerable interest are the 

 intra-ocular transplantations of testicle. As a rule, the testes of newborn 

 animals, in particular those of rats, were used in these experiments, for in- 

 stance, in the work of Pfeiffer, which we have already mentioned, and in the 

 extensive investigations of G. D. Turner; furthermore, in rabbits (Bayer and 

 Wense) conditions seem to be similar. The functioning of these transplants 

 is greatly influenced by hormones ; their presence exerts a stimulation favor- 

 able to spermatogenesis and their absence appears to cause degenerative 

 changes in the more sensitive cells, so that only Sertoli cells survive. In all 

 probability, stimulation by certain hormones is necessary to ensure the survival 

 and function of the most differentiated cellular constituents of endocrine 

 organs, particularly if these organs are under environmental conditions which 

 are not quite adequate. Because the function of the sex organs may inhibit the 

 formation of such stimulating hormones, these transplants may be more suc- 

 cessful in castrated than in normal hosts, and in younger, sexually immature 

 hosts than in older ones. However, it must be mentioned that the very favor- 

 able results of Turner were due to the fact that in addition to the very young 

 age of the donors, donors and hosts were litter mates and the rats belonged to 

 an inbred strain. These conditions eliminated to a great extent the action of 

 unfavorable individuality differentials. But, it seems that after all, the in- 

 dividuality differentials of hosts and transplants were not entirely com- 

 patible in these experiments, and that under less favorable hormonal condi- 

 tions these differences between the differentials could assert themselves and 

 cause an invasion of the transplant by connective tissue and lymphocytes ; at 

 least this is the interpretation which might be given to some of Turner's ex- 

 periments. That the organismal differentials do assert themselves also in the 



