86 THE REALISATION OF THE NUCLEAR FACTORS 



which is necessary for the production of melanin in the pigment 

 cells. In the white axolotl, the production of this . substance 

 would be reduced (Dushane, 1935). (This, at least, is the pre- 

 vailing opinion. Dalton gives a somewhat different explanation 

 of the phenomenon, into which we cannot now enter.) It appear- 

 ed from Baltzer's experiments that merogones consisting of 

 cytoplasm of the black axolotl, and of nuclear material of the 

 white race, had the same type of pigmentation as the white 

 race. Even after early transplantation into a black larva, skin 

 grafts of such a merogone developed a pigmentation of the 

 white type. In this case, the nature of the epidermis, on which 

 the pigmentation depends, appears to be determined entirely 

 by the nuclear factors. 



Mutual comparison of these experiments on heterospermic 

 merogony shows that generalizations with respect to the role 

 of nucleus and cytoplasm in early differentiation are impossible. 

 Specific characters are determined sometimes by the cytoplasm, 

 sometimes by the nucleus alone, and in other cases again by a 

 combination of the two. 



Little is known yet about the nature of the cytoplasmic 

 factors we are discussing here. Some investigators assume that 

 particles occur in the cytoplasm, which can multiply by division, 

 and are in some respects similar to the genes of the nucleus. 

 They are called plasmagenes. Production and multiplication of 

 these plasmagenes would be governed partly by the nuclear 

 genes, partly also by external influences. According to Brachet 

 (1952), they are probably identical with the microsomes 

 (see p. 34). 



We arrive, therefore, at the following conclusion as regards 

 the way in which nuclear factors influence development. As a 

 result of earlier chemodifferentiation, the cytoplasm of certain 

 cells is of such a composition that it can '"activate" certain 

 genes in the nucleus. A reaction then takes place between these 

 genes and the cytoplasmic factors, resulting, for example, in 

 the production of substances which in their turn influence other 

 developmental processes. We must distinguish here between 

 two cases. Firstly, the substances produced as a consequence 

 of gene activity may be restricted to the cells in which they 



