408 THE BIOLOGICAL BASIS OF INDIVIDUALITY 



differences between the individuality differentials of the host and transplant. 

 On the other hand, Caspari and Schwarz believe that concomitant immunity 

 is due to necro-hormones given off by the growing tumor and that the greater 

 immunizing power of a rapidly growing tumor is dependent on the more ex- 

 tensive necrosis which occurs in the central portions of such tumors. 



(2) Immunity following extirpation of a tumor. We have already referred 

 to this kind of immunity as representing a variety of concomitant immunity, 

 which becomes manifest only after extirpation of the first tumor. Uhlenhuth, 

 Haendal and Steffenhagen have shown that when homoiogenous rat sarcomata 

 growing in rats were excised, the rats were thereby rendered immune to re- 

 inoculation with this type of tumor. But, if the operation was incomplete and 

 the tumor recurred, a second inoculation was successful. There has been 

 much discussion concerning this experiment ; some have denied its significance, 

 or even its occurrence. However, Fleisher and the writer were able to confirm 

 the findings of Uhlenhuth and his collaborators ; after extirpation of a homoiog- 

 enous mouse carcinoma No. IX, the animal became immune against reinocu- 

 lation with this tumor. It is to be noted, however, that the growth of this type 

 of carcinoma did not, under normal conditions, lead to the manifestation of a 

 distinct concomitant immunity, presumably because the immune substances 

 are absorbed by the growing tumor. In addition, it must be assumed that the 

 production of immune bodies continues for some time after the source of the 

 antigens has been removed. The absorptive or neutralizing function of a first 

 tumor is not exercised by an autogenous, so-called spontaneous tumor, the 

 extirpation of which does not elicit processes of immunity either against re- 

 inoculation with autogenous or with homoiogenous tumors. We may therefore 

 conclude that the organismal differentials are involved also in these neutraliz- 

 ing mechanisms and that, in particular, homoiogenous individuality differen- 

 tials are able to neutralize homoiogenous immune substances. 



(3) Immunity following retrogression of tumors. Homoiogenous tumors may 

 grow for some time and then retrogress apparently spontaneously; if the 

 tumor pieces used for inoculation have been subjected to chemical or physical 

 injuries previous to transplantation, such a retrogression is particularly apt to 

 occur. In all these cases retrogression takes place because conditions injurious 

 to the tumor cells have had a depressive effect on the tumor growth. We 

 described a spontaneous retrogression of transplanted tumors in 1901. We 

 observed also that during the first stages of retrogression mitotic cell prolif- 

 eration may still proceed quite actively in the tumor cells and that tumors in 

 the early stages of retrogression may be transplanted successfully and may 

 subseqently recover their full vigor of growth ; but at later stages of retro- 

 gression mitotic proliferation is much reduced or it ceases altogether, and 

 from then on the ability of the tumor to recover after renewed transplantation 

 is very much decreased. These observations were subsequently confirmed and 

 extended by Woglom, and in 1905 Clowes and Baeslack established the inter- 

 esting fact that mice, in which a homoiogenous mouse carcinoma had retro- 

 gressed, had become immune against re-inoculation with a homoiogenous 

 tumor. In the case of heterotransplantation, for instance, if a mouse tumor 



