THE REALISATION OF THE NUCLEAR FACTORS 85 



differentiations. This is another indication that the "activation" 

 of the nuclear factors takes place at different times in the 

 different parts of the embryo. In related species of Triton these 

 nuclear factors may be partly identical, and partly different. 

 Normal differentiation will be possible in those cells in which 

 no nuclear factors are activated during differentiation, or only 

 such factors as occur in the maternal species too, and which 

 therefore "fit in" with the protoplasm. Disturbances, sometimes 

 resulting in the death of the cells, will only occur where factors 

 are activated which differ in the two species. Hadorn managed 

 to keep the unimpaired tissues of the merogones alive to much 

 later stages by grafting them into normal embryos. There they 

 differentiated normally. In a few cases, skin ectoderm of such 

 a merogone, when grafted into an embryo of Triton alpestris, 

 remained intact until after the metamorphosis of the host. 

 Meanwhile, the specific characters of this tissue had become 

 visible, and they were found to agree entirely with those of 

 the species which had supplied the cytoplasm, i.e. Triton 

 palmatus. No specific influence of the nucleus of T. cristatus 

 was observed. 



Comparable experiments were made by Dalton (1946) with 

 American newts of the species Triturus torosus, T. similans, 

 and T. rivularis, the same species that were used by Twitty 

 (see p. 80). Here, too, the hybrid merogones die early, before 

 the development of specific differences in pigmentation. But 

 it was again possible to keep the primordia of the pigment cells 

 alive for a longer time by early transplantation into a normal 

 embryo. We have already seen that the pattern of pigmentation 

 depends upon the properties of the pigment cells. Dalton found 

 that it is determined mainly by the nuclear factors, though 

 the cytoplasm has some influence. 



Finally, mention must be made of an experiment by Baltzer 

 (1947) on the pigmentation of merogonic hybrids between black 

 and white axolotls. The difference in pigmentation between 

 these races is caused by a single gene, black being dominant 

 over white. The character does not depend on the pigment cells 

 themselves, but on the epidermis covering them. In the black 

 axolotl, this is said to secrete a substance, probably an oxidase, 



