THE ROLE OF RIBONUCLEIC ACID AND SULFHYDRIL GROUPS 



253 



Figure 9. Dorsal lip from a gastrula blocked after a heat shock (Brachet, 

 1957). 



in Figure 10, the organizer of a heated frog gastrula becomes almost 

 normal again. Differentiation into a notochord, somites, and an archen- 

 teron roof occurs in the graft, as well as induction of a secondary nerv- 

 ous system. 



Cytochemical and biochemical studies of gastrulae which had been 

 submitted to an irreversible or a reversible heat shock have disclosed 

 the fact that the RNA gradients are affected in varying degrees ac- 

 cording to the severity of the treatment. These gradients become very 

 irregular, while mitotic activity stops and the mitotic apparatus de- 

 generates. When a fragment of the irreversibly heated gastrula is 

 grafted into a normal host, the first sign of healing is an impressive re- 

 sumption of nucleolar and cytoplasmic basophilia, which precedes the 

 reappearance of mitotic activity. In the case of reversible heat shocks, 

 the abnormalities found in the distribution of the RNA gradients easily 

 explain why further development becomes abnormal. 



