286 NUCLEIC ACIDS AND GROWTH 3 



since, in experiments on sea urchin eggs, the RNA content seemed to decrease 

 when DNA was synthesized, it was concluded that part of the RNA store is con- 

 verted into DNA. Such a conclusion should now be rejected, because later work 

 by Schmidt et al. (1948), Villee et al. (1949) and Elson et al. (1954) has demon- 

 strated, with improved methods, that the RNA content of sea urchin eggs remains 

 essentially constant during early development. The analytical data obtained by 

 Brachet (1933) were probably inaccurate because RNA was estimated as 

 furfural after acid hydrolysis; it was not realized at the time that the jelly coats, 

 which surround the unfertilized eggs and which dissolve at later stages of develop- 

 ment, produce large amounts of furfural on hydrolysis. 



According to Elson et aL's (1954) most recent estimations for sea urchin eggs, 

 the nucleotide distribution in RNA does not change during development, even 

 when the eggs are treated with LiCl in order to produce morphogenetic abnor- 

 malities. The RNA content, however, varies markedly during morphogenesis : an 

 initial rapid drop occurs after fertilization, then a first rise is observed during 

 cleavage; a second rise is found just before the onset of gastrulation, and might be 

 associated with protein synthesis : the importance of gastrulation, as the initial 

 stage of new protein synthesis, has been emphasized by several workers (Brachet, 

 i94ga; Perlmann and Gustafson, 1948; Gustafson and Hasselberg, 1951). 



According to our co-worker Steinert (1951, 1953), the RNA content of Amphib- 

 ian eggs remains essentially constant until gastrulation : it then steadily increases, 

 presumably in connection again with increased protein synthesis. Inhibition of 

 development by metabolic poisons like dinitrophenol or usnic acid is accompanied 

 with a block in RNA synthesis; both phenomena are simultaneously reversible. 



Isotope experiments by Villee et al. (1949) and by Abrams (1951) have further 

 indicated that, in sea urchin eggs, RNA cannot be the precursor of DNA. Accord- 

 ing to Abrams (1951), the DNA purines synthesized during development have 

 10 times the specific activity of those of RNA, which are therefore ruled out as 

 precursors. Abrams (1951) finds, however, that the great majority of the DNA 

 synthesized by sea urchin eggs (70-81%) comes from a precursor pre-existing in 

 the egg; this precursor might or might not be RNA. It might even be that all of 

 the synthesized DNA comes from pre-existing precursors, because insufficient 

 caution has been taken in Abrams' ( 1 95 1 ) experiments to avoid contamination of 

 the embryos by other organisms (bacteria and unicellular algae). Growth of these 

 organisms, which are difficult to separate from the eggs, can be extremely rapid 

 as soon as even a small fraction of the eggs are cytolysed during culture : it is en- 

 tirely possible that all of the tagged DNA found by Abrams ( 1 95 1 ) comes from such 

 contaminations. 



If we exclude RNA as a precursor of DNA, from where does the latter originate? 

 Recent experiments by Hoff-Jorgensen (1954) and by Elson et al. (1954) indicate 

 that the unfertilized sea urchin eggs contain a DNA reserve in the cytoplasm: the 

 microbiological techniques used by these workers indicate that the sea urchin eggs 

 contain some excess DNA, amounting to 5-15 times the diploid DNA content. 

 According to Hoff-Jorgensen (1954), the DNA content in the sea urchin embryo 

 remains constant from the unfertilized egg to the i6-cell stage. 



Similar results have been obtained, again with sea urchin eggs, by Marshak and 



