68 GROWTH AND MORPHOGENESIS 



ment of eggs which contain an abnormal paternal nucleus (lethal 

 hybrids) or gastrulae which have been submitted to a heat shock. 

 The most likely explanation for this revitalization phenomenon is 

 the same in all these cases. The fragments of the blocked gastrula 

 can no longer synthesize certain substances, RNA in particular, 

 and its development is thus arrested. If it is grafted into a normal 

 gastrula, substances of an unknown nature, but which are re- 

 quired for RNA synthesis, diffuse from the host to the graft. 



In chick embryos, inhibitors of RNA synthesis also impair mor- 

 phogenesis. For instance. Fox and Goodman (1953) found that 

 abnormal synthetic nucleosides, in which ribose was replaced by 

 another sugar (e.g. glucose), inhibit the development of explanted 

 chick embryos. Waddington et al. (1955) found that the regions 

 which are most sensitive to chemical analogues such as benzimid- 

 azole or azaguanine are precisely those which show the highest 

 incorporation of methionine into proteins. Once more, RNA syn- 

 thesis, protein synthesis and morphogenesis appear as very closely 

 linked in developing eggs. 



Finally, Hisaoka and Hopper (1957) have been working on zebra 

 fish eggs and have used barbituric acid as a tool for experimentation. 

 They concluded from these studies that there is a link between mor- 

 phogenesis and RNA synthesis in fish eggs as well as in those of the 

 amiphibians and the chick. 



Substances other than purine and pyrimidine analogues have 

 comparable effects. For instance, the well-known inhibitors of 

 oxidative phosphorylation, dinitrophenol and usnic acid, completely 

 inhibit morphogenesis. The inhibition can be largely reversed if the 

 treated eggs are returned to normal medium. However, abnormal- 

 ities (persistent yolk plug, microcephaly) can often be found in 

 these cases (Brachet, 1954). Cytochemical (Brachet, 1954) and 

 quantitative (Steinert, 1953) studies have clearly shown that inhibi- 

 tion of development and RNA synthesis always go hand in hand. 

 When the dinitrophenol-treated embryos are returned to normal 

 medium, RNA synthesis is resumed, but only if morphogenesis 

 also takes place. 



Another group of interesting substances is that of the steroid 



