EXPERIMENTAL MODIFICATIONS 71 



fertilized, but still uncleaved, eggs are centrifuged. If these eggs 

 cleave normally, the blastoporal lip forms in a normal position. 

 But the material which invaginates and which corresponds to the 

 organizer is much poorer in RNA than is the organizer in normal 

 eggs. This reduction in the RNA content of the invaginated mate- 

 rial is accompanied by a marked decrease in its inducing activity. 

 The hypomorphoses described by Pasteels (1940) are the logical 

 result of such a situation. 



Very different results are obtained when blastulae are centrifuged. 

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

 of RNA-rich material at the centrifugal pole of the cells. Later on, 

 foci of strong RNA synthesis, which are characterized by their very 

 strong basophilia, make their appearance (Fig. 25, p. 64). Finally, 

 an accessory nervous system forms in each of their basophilic areas. 



Further investigations, with autoradiography as the major tool, 

 are required in order to analyze further the connections existing 

 between RNA distribution and synthesis on one hand and mor- 

 phogenesis on the other hand in centrifuged eggs. There is no doubt, 

 however, that the present evidence confirms the view that RNA and 

 morphogenesis are intimately Hnked processes. 



The same conclusion can be drawn from experiments in which a 

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

 ing for one hour at temperatures ranging from 36° to 37° according 

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

 mild heat shock produces a reversible inhibition of development. 

 When development again proceeds, numerous malformations are 

 found. These abnormalities are essentially similar to those pro- 

 duced 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 2-3 days 

 (Fig. 26, p. 73). If a piece of the blocked gastrula is placed in con- 

 tact with cells of a normal gastrula, even when they belong to an- 

 other species, a dramatic "revitahzation" of the heated cells occurs. 

 As shown in Fig. 27 (p. 74), the organizer of a heated frog gastrula 

 becomes almost normal again. Differentiation into notochord, 

 somites and archenteron roof occurs in the graft, as well as induc- 



References p. 90/93 



