THE ROLE OF RIBONUCLEIC ACED AND SULFHYDRIL GROUPS 



247 



to the important conclusion that new RNA is being synthesized in mor- 

 phogenetically active regions during diflFerentiation. 



It should be pointed out, however, that gradients similar to those 

 just described for RNA have been detected and described for other 

 substances as well, such as -SH groups bound to the proteins, to 

 which we shall return (Brachet, 1940). In particular, the distribution 

 of reducing systems ( Piepho, 1938; Fischer and Hartwig, 1936; Child, 

 1948) follows the now-familiar pattern of dorso- ventral and antero- 

 posterior gradients. The same pattern has also been found in oxygen 

 consumption (Sze, 1953), the incorporation of amino acids into pro- 

 teins (Ealcin et al., 1951), and the incorporation of labeled carbon 

 dioxide into nucleic acids and proteins ( Flickinger, 1954 ) . 



Since mitochondria play a leading part in all processes linked to 

 energy production, it appears likely that these cell organelles are dis- 

 tributed, together with the microsomes, along gradients which super- 

 impose themselves on the morphogenetic gradients. One can therefore 

 hardly avoid the conclusion that the latter represent regions where the 

 yolk reserves are transformed into "true" cytoplasm ( ergastoplasm and 

 mitochondria) at a faster rate than elsewhere. Looking at them from 

 another angle, these gradients may be considered as gradients in the 



Figure 5. Distribution of RNA in a young amphibian neurula (Brachet, 

 1957). 



