THE ROLE OF RIBONUCLEIC ACID AND SULFHYDRIL GROUPS 



245 



Distribution of RNA in normal amphibian eggs 



Precise observations, because of the quality of the available cyto- 

 chemical methods, can be made in the case of RNA distribution during 

 development of the amphibian egg ( Brachet, 1942, 1944 ) . As shown in 

 Figure 2, a polarity gradient is already visible in the unfertilized or 

 freshly fertilized eggs; it decreases from the animal to the vegetal pole 

 and remains intact during cleavage ( Figure 3 ) . At gastrulation ( Figure 

 4), a secondary RNA gradient, decreasing from dorsal to ventral, super- 

 imposes itself upon the initial animal-vegetal gradient. As a result of 

 RNA synthesis and morphogenetic movements, the two gradients inter- 



Figure 2. Distribution of 

 RNA in a fertilized amphib- 

 ian egg (Brachet, 1957). 



Figure 3. Distribution of 

 RNA in an amphibian blas- 

 tula (Brachet, 1957). 





