THE REALISATION OF THE NUCLEAR FACTORS 81 



in the dorsal regions of the sides, above the muscle segments, 

 whereas in T. rivularis they become evenly scattered over the 

 whole flank. The differences in pigmentation, therefore, are 

 due to differences in the migrations of the pigment cells. These 

 in their turn might depend either upon differences in the nature 

 of these cells, or upon differences in the orienting and attract- 

 ing influences from the environment to which they are exposed. 

 By means of transplantation experiments, Twitty and his 

 collaborators proved, indeed, that both the skin epithelium and 

 the muscle segments of various species have different properties 

 in this respect. But the specific differences in the pattern of 

 pigmentation proved to be due mainly to the properties of the 

 pigment cells themselves. In T. torosus, the latter are strongly 

 attracted by the muscle segments, and to a lesser degree by 

 the central nervous system. The pigment cells of T. rivularis 

 lack this property ; hybrids possess it to a smaller extent. Parts 

 of the pigment cell primordia in the neural fold of torosus 

 embryos were replaced by corresponding parts of rivularis 

 embryos, and vice versa. In this case, the pigment cells of the 

 torosus graft accumulated over the muscle segments of the 

 rivularis host, whereas the pigment cells of the rivularis graft 

 scattered evenly over the side of the torosus host. In other 

 words, the graft's own specific pattern will develop even in the 

 foreign host (PL V d-e) *. Twitty (1945) has shown in later 

 experiments that the dorsal accumulation of the torosus melano- 

 phores is a secondary phenomenon. As soon as they have reach- 

 ed a certain degree of differentiation, the cells withdraw their 

 processes, thereby moving closer together. In T. rivularis, the 

 melanophores always remain at a lower level of differentiation ; 

 they do not become so strongly pigmented, and do not show 

 secondary accumulation. The nuclear factors, therefore, are 

 directly responsible for a difference in the ultimate level of 

 differentiation of cells of this category; indirectly, this results 

 in a modification of the pattern of skin pigmentation. 



In the cells of hybrids, the paternal nuclear material is always 

 combined with the maternal chromatin of the egg nucleus. This 

 means that the influence of the paternal genes on the foreign 



* Facing page 96. 



Raven - Outline Physiologie 6 



