72 GROWTH AND MORPHOGENESIS 



tion of a secondary nervous system. The inductive power of the 

 heated organizer is, however, subnormal, because it has lost the 

 power of inducing a head, but has retained that of spinocaudal 

 induction (Takaya, 1955). "Revitalization" of ectoblast cells from 

 a heated gastrula can also be demonstrated when the cells are placed 

 in contact with a normal organizer. These cells can still react to in- 

 ducing stimuli by differentiation into a neural plate. 



Cytochemical and biochemical studies of gastrulae which have 

 been submitted to an irreversible or a reversible heat shock have 

 disclosed the fact that the RNA gradients are affected to varying 

 degrees according to the severity of the treatment. These gradients 

 become very irregular, while mitotic activity stops and the mitotic 

 apparatus degenerates. When a fragment of the irreversibly heated 

 gastrula is grafted into a normal host, the first sign of healing is an 

 impressive resumption of nucleolar and cytoplasmic basophilia, 

 which precedes the reappearance of mitotic activity. In the reversible 

 heat shocks, the abnormalities found in the distribution of the RNA 

 gradients easily explain why further development becomes ab- 

 normal. 



Quantitative estimations of the RNA content (Steinert, 1951) 

 have shown that, as suggested by cytochemical observations, RNA 

 synthesis is completely inhibited in the irreversibly blocked gastru- 

 lae ; if the heat shock is too severe, cytolysis begins after 3 days and 

 the RNA content begins to drop. Steinert's (1951) results have been 

 confirmed by Hasegawa ( 1 955) who demonstrated that in amphibian 

 eggs RNA synthesis is much more sensitive to heating than DNA 

 synthesis. This finding of Hasegawa (1955) is again in excellent 

 agreement with the cytochemical findings. 



There is thus no doubt that the ribonucleoprotein structures of 

 the amphibian egg cytoplasm are particularly sensitive to heating. 

 In fact, Brachet (1949a) was able to show that a large proportion 

 of the RNA which is normally found in the microsome fraction 

 appears in the supernatant liquid when heated gastrulae are homog- 

 enized and submitted to differential centrifugation. It would, 

 however, be unwise to believe that heat shocks act in a specific way 

 on RNA-containing structures. At the gastrula stage, these shocks 



Text continued on p. 77 



