356 THE BIOLOGICAL BASIS OF INDIVIDUALITY 



it is of importance to note in this connection that cells of a less differentiated 

 nature composed these tumors. Moreover, different types of fowl tumors 

 differed in their ability to grow in heterogenous hosts and, likewise, different 

 races of certain avian species differed in their suitability as hosts. The transfer 

 of the tumor by large amounts of tumor cells was, on the whole, more suc- 

 cessful than the transfer by means of virus-containing tumor extracts. In 

 general, fowl sarcoma could be transmitted much more readily to newly 

 hatched heterogenous birds than to somewhat older ones. 



According to the quite recent experiments of Duran-Reynals it can be 

 shown that if we transfer Rous fowl sarcoma I to ducklings, at first the un- 

 changed chicken tumor virus or cells cause the tumor development in these 

 ducklings, as indicated by the fact that the tumors thus produced have not 

 only the same morphological characteristics as the original chicken tumor, but 

 they also possess the same tissue affinities and the same tendency to develop 

 in certain regions of the host. However, after the tumors have lived for some 

 time in ducklings, they may change their characteristics, the chicken-adapted 

 virus becoming duck-adapted virus; it tends to cause sarcomas in organs, 

 different from those in which it grew at first, growing now in bones or 

 lymph glands, calling forth a lymphosarcoma in the latter organ. Further- 

 more, this changed tumor tends to become generalized. A similar adaptation 

 seems to occur if tumor cells are inoculated ; these also cause the same kind of 

 tumors as the duck-adapted virus. Such a duck-adapted virus or cell suspen- 

 sion induced not only tumor formation in ducklings, but also in adult ducks. 

 Cells and viruses are now no longer heterogenous but homoiogenous elements 

 for the duck. If such duck-adapted virus or cells suspension is transferred 

 back into chicken, it seems at first to behave like material heterogenous for the 

 chicken, but after some time, the duck-adapted virus or cell suspension can 

 again become chicken-adapted, being thus converted into a homoiogenous 

 virus for the chicken. However, it should not be concluded from these ex- 

 periments that a chicken cell was actually transformed into a duck cell, but it 

 seems, merely, that the changed virus altered secondarily the tumor-producing 

 characteristics of the chicken cells in which the virus lived. 



We have seen that it is possible to a certain extent to protect transplanted 

 normal tissues against the injurious reactions which ordinarily take place in 

 the host, and by various means to diminish the reactions in the host which 

 follow as a rule transplantation of strange tissues. Thus homoiotransplants 

 induced less active reactions when younger hosts were used ; also when they 

 were made into the anterior chamber of the eye, into the brain, or after pre- 

 ceding injections of trypan blue. More striking differences have been observed 

 when similar methods were employed in the case of transplanted tumors, and, 

 indeed, experiments with tumor transplants preceded experiments of a like 

 nature with normal tissues. It has been found by Murphy that transplanted 

 heterogenous tumors, even if host and tumor were phylogenetically far distant, 

 grew for some time on the chorio-allantoic membrane of the chick; an ex- 

 periment with heterogenous embryonal tissue also succeeded; but as soon as 

 the development of the chick embryo had reached the stage at which the 



