THE REALISATION OF THE NUCLEAR FACTORS 79 



Bufo communis '^ ^ ^- viridis (f , whereas in the reciprocal 

 cross the hybrids develop poorly, and in the cross Rana escu- 

 lenta X R. fusca development ceases already at the gastrula 

 stage. Here we also find so-called "false hybrids", in which the 

 penetrating sperm has set development going, but does not 

 otherwise take part in the process (e.g. in Rana esculenta X 

 Bufo viridis). 



J. A. Moore (1941) studied the development of a great 

 number of American frog hybrids. In many cases, these devel- 

 oped normally until the early gastrula stage, but then develop- 

 ment ceased; the rate of development was entirely similar to 

 that of the parent species which had produced the eggs. In 

 hybrids in which development proceeded further, its rate was 

 again at first determined exclusively by maternal factors. How- 

 ever, with the appearance of the first organ primordia after 

 the completion of the gastrula stage, the influence of the 

 paternal chromosomes on the rate of development became 

 noticeable. 



Further it has been found in frog hybrids which stopped 

 developing at the gastrula stage, that simultaneous deviations 

 occurred in respiration and carbohydrate metabolism (Barth 

 and Jaeger), and the synthesis of ribonucleic acid which nor- 

 mally begins at this stage dit not set in (Brachet, 1952). 

 Evidently the presence of the foreign chrom.osomes inhibits 

 certain processes by which the nucleus influences the cell meta- 

 bolism (cf. p. 65). Curiously enough, parts of such "lethal 

 hybrids", grafted into normal embryos, proved able to continue 

 their development more or less normally. This is accompanied 

 by a distinct synthesis of ribonucleic acid in the grafts. This 

 "revitalisation" of the lethal tissues must probably be ascribed 

 to the fact that the substances indispensable to metabolism, 

 which the cells of the hybrid cannot themselves synthesise, are 

 now supplied by diffusion from the environment. 



The influence of the paternal nuclear factors on further 

 development has been carefully studied in some newt crosses. 

 Hamburger (1936) crossed Triton taeniatus and T. cristatus. 

 The hybrid larvae initially were the same size as those of the 

 maternal species. Later, however, differences developed. T. 



