THE ROLE OF RIBONUCLEIC ACID AND SULFHYDRIL GROUPS 251 





- /• H.>vv-V.;- 





Figure 6. Stratification of a fertilized egg after centrifugation. From top to 

 bottom: fat; RNA-rich hyaloplasm; pigment and yolk (Brachet, 1957). 



Very different results are obtained when blastulae are centrifuged. 

 First there is a collapse of the blastocele roof and an accumulation of 

 RNx\-rich material at the centrifugal pole of the cells. Later on, foci 

 of strong RNA synthesis, characterized by very strong basophilia, make 

 their appearance (Figures 7 and 8). Finally, an accessory nervous 

 system forms in each of these basophilic areas. Present evidence thus 

 confirms the view that RNA and morphogenesis are intimately linked. 



The same conclusion can be drawn from experiments in which a 

 young amphibian gastrula is submitted to a "heat shock" ( i.e., heating 

 for one hour at temperatures ranging from 36^ to 37° C, according to 

 the species). We have shown (Brachet, 1948, 1949 a, b) that a mild 

 heat shock produces a reversible inhibition of development; when the 

 latter proceeds again, numerous malformations are found. These ab- 

 normalties are essentially similar to those produced by treatment with 

 lithium chloride. 



If the temperature chosen is a little higher, the block in develop- 

 ment is irreversible, but no cytolysis can be detected for two or three 

 days (Figure 9). If a piece of the blocked gastrula is placed in contact 

 with cells of a normal gastrula, even when they belong to another 

 species, dramatic "revitalization" of the heated cells occurs. As shown 



