V NUCLEIC ACIDS EMBRYONIC DEVELOPMENT 287 



Marshak ( 1 953) , who used an isotope dilution technique ; but, after further analysis, 

 Marshak and Marshak (1954a, 1955) have arrived at very different conclusions. 

 Taking into account the presence of contaminating ovarian cells and polar bodies, 

 they come to the rather unexpected view that sea urchin and starfish eggs contain 

 no DNA at all! A startling consequence of these findings is that DNA cannot have 

 a primary genetic role. According to Marshak and Marshak ( 1 955) , DNA probably 

 plays a regulating role and competes with RNA in cellular metabolism. Marshak 

 and Marshak (1954b) find further evidence for the absence of any DNA in sea ur- 

 chin eggs in the fact that ripe unfertilized eggs contain 96.5% of their thymine in 

 an acid-soluble form : there apparently is a breakdown of the ovarian DNA at the 

 time of maturation. Finally, Marshak and Marshak (1954a, 1955) have repeatedly 

 emphasized the fact that the egg nucleus, i.e. the female pronucleus, is Feulgen nega- 

 tive and therefore contains no DNA. 



It is diflfiicult at the present time to draw definite conclusions from such con- 

 flicting evidence; there is no doubt that Marshak and Marshak (1954a, 1955) 

 are right when they emphasize the importance of contaminating ovarian cells 

 and polar bodies. As a matter of fact, the same point was made long ago by 

 Brachet (1944a) when criticizing an early claim by Blanchard (1935) that DNA is 

 present in large amounts in unfertilized sea urchin eggs. On the other hand Mar- 

 shak and Marshak's (1954a, 1955) conclusions are based on calculations assuming 

 that 3 polar bodies are eliminated, without giving cytological evidence for this 

 opinion: in many species, only 2 polar bodies are eliminated, the first one re- 

 maining uncleaved. Finally, the statement that the female pronucleus is Feulgen 

 negative cannot remain unchallenged : it is true that no Feulgen staining material 

 could be found by Brachet (1933), nor by Mirsky and Ris (1949). But recent 

 unpublished observations have convinced the author that the nucleus of unfer- 

 tilized sea urchin eggs actually contains a shell of Feulgen positive material, just 

 under the nuclear membrane; the centre of the nucleus remains unstained. These 

 observations have been made repeatedly on two different species, using the best 

 available conditions for fixation, staining and microscopical observation. Similar 

 findings, on another species of sea urchin, have also been recently reported by 

 Burgos (1955). 



In conclusion, the author is convinced that sea urchin eggs contain DNA in 

 their nucleus, just as any other cell, and that Marshak and Marshak's (1954a, 

 1955) sweeping conclusion about the genetic role of DNA should be regarded 

 with considerable scepticism. On the other hand, their work clearly indicates that 

 sea urchin eggs contain very little, if any, DNA store. 



The situation is also very puzzling when other species are investigated for the 

 presence of a DNA store in unfertilized eggs: in frog eggs (HoflT-Jorgensen and 

 Zeuthen, 1952; HoflT-Jorgensen, 1954), the DNA content is said to remain constant 

 until the late blastula stage, where rapid synthesis occurs. According to the Danish 

 authors, the unfertilized egg contains enough DNA, as an initial reserve, for about 

 5,000 cells. This reserve is much larger in eggs of hens (HoflT-Jorgensen, 1954), 

 since no DNA is synthesized until the fourth day of incubation, when the embryo 

 contains ca. 5* 10'' diploid cells. 



It is obvious that the true significance of these results entirely depends on the 



Literature p. 299 



