THE GERM-PLASM THEORY 383 



demonstrable that there may be two alternative sets of homologous 

 determinants present in a cell, but that on- any occasion only one 

 of these becomes active, a fact which we can only explain on the 

 assumption that only one of these is affected by the specific liberat- 

 ing stimulus. The phenomena of regeneration, of polymorphism, of 

 germ-cell formation, &c., compel us to the assumption that numerous 

 cells, even after the completion of the building up of the body, contain 

 two or more kinds of determinants, as in a sense inactive ' accessory 

 idioplasm,' each of which could control the cell alone, though in reality 

 it only does control it when it is affected by the appropriate liberating 

 stimulus. I stated this, view some years ago when I attempted to 

 define more precisely the role played by ' external influences as 

 developmental stimuli ^ '. It is not, then, that I underrate the 

 importance of external influences on the organism!, but I believe that 

 a still larger part of the determination of what shall happen at a 

 particular point depends on the primary constituents, and that these 

 are not alike at all parts of the body. 



All living processes, therefore, both those of growing and of 

 differentiation, depend always upon the interaction of external and 

 internal factors, of the environment and the living substance, and the 

 resultants of the interaction, namely, the structure of the body and its 

 parts must necessarily turn out differently, not only when the germ- 

 substance is different, but when the essential conditions of development 

 are changed. But that the constitution of the germ is by far the 

 most potent factor, and that the nature of the results of development 

 depends on it in a much greater degree than on the external conditions, 

 has long been known. The conditions, such as warmth, may vary 

 within certain limits, and yet the frog's egg becomes a frog ; though 

 it does not follow that the result of development may not be modified 

 through certain changes in the conditions. The interesting experi- 

 ments made by Herbst with the. eggs of sea-urchins have shown that, 

 in artificially altered sea-water in which sodium-salts are to a slight 

 extent replaced by lithium-salts, these eggs develop into larvae which 

 only remotely suggest the normal structure, and diverge widely from 

 it both in external shape and in the form of the skeleton. 



Such larvffi are not able to survive, but soon perish; they 

 are, however, of great interest from the point of view of our theory, 

 for they show that determinants do not bring forth the same structure 

 under all circumstances, but that, as I have already said, they are 

 vital units of specific composition, which play a part in the course of 



' Aussere Einjlusse als Entwioklungsreize [External Influences as Stimuli to Develop- 

 ment], Jena, 1894. 



