V NUCLEIC ACIDS EMBRYONIC DEVELOPMENT 289 



gradients in the blastulae, gastrulae and neurulae has been confirmed by quanti- 

 tative estimations (Brachet, 1941b; Steinert, 1951; Takata, 1953; Chantrenne 

 and Brachet, 1946). 



It should be added that these gradients are strikingly parallel to the hypothet- 

 ical morphogenetic gradients of the experimental embryologists; it would, how- 

 ever, be a mistake to believe that morphogenetic gradients are nothing else but 

 RNA gradients, since a similar distribution has been found for many other sub- 

 stances: SH-groups linked to proteins (Brachet, 1940), reducing power (Fischer 

 and Hartwig, 1936; Piepho, 1938; Child, 1948) alkaline phosphatase (Krugelis, 

 1947), oxygen consvimption (Sze, 1953), dipeptidase (Barth and Sze, 1953). On the 

 other hand, there is no dorso-ventral gradient, at the gastrula stage, for total pro- 

 teins and lipids (Barth and Sze, 1953; Gregg and Lovtrup, 1950). Since we know 

 that RNA and SH-groups are associated mostly with small cytoplasmic granules, 

 the microsomes, while respiratory enzymes are concentrated in the mitochondria, 

 we can hardly escape one conclusion: that the biochemical gradients which are 

 parallel to the morphogenetic gradients are, in fact, gradients in the distribution 

 of cytoplasmic particles. 



If RNA synthesis and distribution in gradients are important morphogenetic 

 factors, one would expect experimental alterations of RNA metabolism to inter- 

 fere with morphogenesis : this has been found to be the case in all instances studied 

 so far. 



It has been shown, for instance, that chemical analogues of purines and pyri- 

 midines, which slow down nucleic-acid synthesis, completely stop morphogenesis 

 at stages which are characteristic for each compound (Brachet, 1944a, 1946; 

 Bieber, 1954, for amphibian eggs; Fox and Goodman, 1953; Waddington et al., 

 1955, for chicken embryos). In most cases, the effect is reversible when the embryos 

 are placed in normal medium; but, in the chicken, inhibition by the purine 

 analogue azaguanine is only reversed on the addition of hypoxanthine. Further- 

 more, the regions of the embryo which are most sensitive to the inhibitors are 

 generally those where incorporation of labelled amino acids into the proteins is 

 most active (Waddington et al., 1955). Similar results have been obtained with 

 acriflavine (Brachet, 1946), which precipitates nucleic acids in vitro: reversibility 

 of development is improved by addition of RNA or adenylic acid. (For further 

 data on analogues and growth see Chapter 5). 



As already mentioned before, poisons of oxidative phosphorylations like dinitro- 

 phenol and usnic acid inhibit RNA synthesis and morphogenesis simultaneously in 

 Amphibians; if development is allowed to proceed by removal of the inhibitor, 

 RNA synthesis is resumed, and it runs parallel with morphogenesis (Steinert, 1 953) . 



Another interesting case is that of female sex hormones, which, according to 

 Cagianut (1949), affect the RNA distribution in amphibian eggs; the RNA gra- 

 dients are altered during early cleavage and, as a consequence, development 

 becomes highly abnormal and leads to asymmetrical embryos. It is noteworthy 

 that addition of yeast RNA definitely improves the development. 



Physical agents are no less interesting, in this respect, than chemical substances: 

 for instance, centrifugation of just fertilized eggs strongly modifies the initial RNA 

 polarity gradient and leads to more or less complete failure of development 



Lileralure p. 2gg 



