EXPERIMENTAL MODIFICATIONS 67 



occur during the early development of mammalian eggs, and that, 

 as in other vertebrates, RNA synthesis is especially marked in the 

 mesoblast and the induced parts of the ectoblast. 



In short, the cytochemical data obtained in the case of fishes, 

 reptiles, birds and mammals agree very well with the general con- 

 clusions which we have drawn from the study of amphibian eggs. 

 RNA is accumulated and is most actively synthesized in the regions 

 of the embryo which have the greatest importance for morphogenetic 

 processes. 



We shall now try to answer another important question: what 

 happens to the ribonucleoprotein gradients when morphogenesis 

 or RNA synthesis are experimentally modified ? 



4. EXPERIMENTAL MODIFICATIONS OF RNA SYNTHESIS 



AND morphogenesis: EFFECTS ON RNA GRADIENTS 



IN AMPHIBIAN EGGS 



If synthesis of RNA along animal and vegetal gradients is really an 

 essential factor in morphogenesis, inhibition of RNA synthesis by 

 treatment with chemical analogues of purines and pyrimidines should 

 lead to the cessation of development or to abnormal development. 

 This expectation has been fulfilled, as was first shown by Brachet 

 (1944) in the cases of barbituric acid, benzimidazole and acriflavine. 

 These early studies have been considerably extended by Bieber 

 (1954), Bieber and Hitchings (1955) and Liedke et al. (1954, 

 1957a, b), who used a considerable number (more than one hun- 

 dred) of chemical analogues of purines, pyrimidines and nucleo- 

 sides. They usually found inhibition of development at a definite 

 stage. This fact suggests the possibility that new enzymatic mecha- 

 nisms for RNA synthesis appear at definite stages of development. 

 Of special interest is the fact observed by Liedke et al. (1957b) 

 that some of the analogues which were used blocked development 

 at the gastrula stage. If a piece of the arrested gastrula was grafted 

 into a normal gastrula, the blocked fragment resumed normal 

 development and differentiation. We shall find later other examples 

 of such "revitalization" phenomena, when we discuss the deyelop- 



References p. 90/93 . . v^\ A (. 



