SUBSTRATUM AND MORPHOGENIC SUBSTANCES 453 



host organism, but at an earlier or later date, in accordance with its sufficient 

 or insufficient sensitization. Yet, within a certain range of sensitization the 

 hormone active in the host at the time of metamorphosis tends to induce 

 metamorphosis in the transplant synchronously with that of the host tissues. 



As to the significance of organismal differentials in the process of meta- 

 morphosis, in urodele amphibia the effects described can be noted after auto- 

 and homoio-, as well as after heterotransplantation of the respective tissues, 

 but in anuran amphibia W. Schultz observed that only auto-, but not homoio- 

 transplanted skin takes part in metamorphosis. With the progress in phylo- 

 genetic development, the tissues become more and more specialized and the 

 organismal differentials more selective, so that only if the latter are nearly 

 related in the host and the transplant is the metabolism of the transplanted 

 tissue such that the graft is able to respond effectively to the metamorphosing 

 substances. 



In the experiments cited in this chapter certain tissue reactions may depend 

 upon the interaction between several sets of factors: namely, (a) conditions 

 inherent in the tissues and determining their ability to undergo certain 

 changes ; (b) the action of hormone-like distance substances circulating in 

 the bodyfluids of the host, combining with the transplanted tissues and thus 

 causing their specific effects; (c) the time when these hormone-like sub- 

 stances act on the tissues. In some cases substances of this kind must act 

 not only following but also previous to transplantation, and thus sensitize the 

 tissues; (d) the action of organismal differentials affecting, as a rule, the 

 transplanted tissues and, much less or not at all, the hormone-like substances, 

 although in some cases hormones may possess some of the less specific or- 

 ganismal differentials ; (e) mechanical factors acting as stimuli in combination 

 with sensitizing agents; as in the formation of maternal placentomata ; (f) 

 a balancing action between conditions inherent in the tissues and the effects 

 of the hormone-like distance substances. 



A balancing action, similar to the last mentioned factor, may take place in 

 ordinary transplantation of tissues. In this case we have in all probability to 

 deal with an antagonism between factors inherent in the transplant and con- 

 tact substances residing in the tissues of the host which serve as a soil for 

 the transplant. Thus in the experiments of Milojevitch, with transplantation 

 of regenerative buds of extremities, the surrounding host tissues apparently 

 determined what kind of limb was to develop, their influence dominating over 

 the conditions inherent in the transplants ; on the contrary, in the experiments 

 of Ruud, with transplantation of embryonal limb buds, factors inherent in the 

 transplants determined the result. 



In this connection we may also refer to Goldschmidt's theory of the 

 mechanisms underlying Mendelian heredity, and in particular to his interpreta- 

 tion of the sex-intergrades which result from hybridization of different races 

 of Lymantria. In the various types of hybridizations there takes place a 

 mutual interaction of germ cells possessing different genetic constitutions and 

 therefore also different precursors of organismal differentials. Goldschmidt 

 attributes different potencies to various allelomorph genes or gene combina- 



