80 THE REALISATION OF THE NUCLEAR FACTORS 



cristatus grows more rapidly than does taeniatus. The hybrids 

 are intermediate, though cristatus X taeniatus grows some- 

 what faster than the reciprocal hybrid. This difference between 

 the hybrids shows that growth does not depend only on the 

 nuclear factors, but on the cytoplasmic factors as well: the 

 composition of the nuclei is identical in the two hybrids, but 

 their cytoplasm is different, being almost exclusively maternal. 

 Another difference between the parent species is that the white 

 pigment cells on the caudal fin appear earlier in Triton cristatus 

 than in taeniatus, and have somewhat different patterns in the 

 two species. In both hybrids, the appearance of this character 

 coincides with that in cristatus, and at that time the pattern, 

 too, essentially tallies with that of cristatus (Pl.IVa-d) *. Later, 

 however, it begins to approach the taeniatus pattern. The 

 similarity in behaviour of the two hybrids on this point shows 

 that cytoplasmic factors do not play a role here, but that the 

 formation of these pigment cells is governed entirely by the 

 nuclear factors. Finally, there are marked differences between 

 taeniatus and cristatus larvae in the structure of the fore-limb, 

 viz. in shape and length of the fingers. The limbs of the hybrids 

 are intermediate between those of the parent species (PI. IV 

 e-g). There is no difference between reciprocal hybrids on this 

 point, so that here again only nuclear factors seem to play a role. 



In a study of American species of the genus Triturus, Twitty 

 (1936) was able to carry the analysis still further by means of 

 a combination of transplantations and hybridisations. There are 

 characteristic differences in pigmentation between the species 

 studied by him. In Triturus torosus, the black pigment cells 

 form a well defined black band on each side along the base 

 of the dorsal fin. In T. rivularis, they are dispersed all over 

 the sides of the body. The pigmentation of hybrids between 

 these species is more or less intermediate between that of their 

 parents. Evidently the distribution of the pigment cells over 

 the sides is governed by the nuclear factors (PI. V a-c). 



Twitty tried to find out how the nuclear factors influence the 

 distribution of the pigment cells. These cells originate dorsally, 

 from the embryonic neural folds (cf p. 98), and spread from 

 there over the sides of the body. In T, torosus, they accumulate 



* Facing page 49. 



