E. HOFF-J0RGENSEN 



REFERENCES 



Bragg, A. N. (1938). The organization of the early embryo of Bufo cognatus. %. 



Zjellforsch. 28, 154-178. 

 Elson, D. and Chargaff, E. (1952). On the DNA content of sea-urchin gametes. 



Experientia 8, 143-149. 

 Friedkin, M. and Kalckar, H. M. (1950). Desoxyribose-i -phosphate. J. biol. 



Chem. 184, 437-464. 

 Hoff-Jgrgensen, E. (1952). A microbiological assay of deoxyribonucleosides and 



deoxyribonucleic acid. Biochem. J. 50, 400-403. 

 Hoff-Jorgensen, E., Moustgaard, J. and Moller, P. (1952). The content of 



B-vitamins in some ordinary Danish feedstuffs. Acta agric. Scand. 2, 305-31 1. 

 Hoff-Jorgensen, E. and Zeuthen, E. (1952). Evidence of cytoplasmic deoxyribo- 



sides in the frog's egg. Nature, Lond. 169, 245. 

 Kunitz, M. (1950). Crystalline desoxyribonuclease. J. gen. Physiol. 33, 349-377. 

 McNutt, W. S. (1952). The enzymatically catalysed transfer of the desoxyribosyl 



group from one purine or pyrimidine to another. Biochem. J. 50, 384-397. 

 Mirsky, A. E. and Ris, H. (1949). Variable and constant components of chromo- 

 somes. Nature, Lond. 163, 666-667. 

 Rugh, R. (1948). Experimental embryology. Burgess Pub. Co. Minneapolis, p. 126. 

 Schmidt, G. and Thannhauser, S.J. (1943). Intestinal phosphatase. J. biol. Chem. 



*49> 369-385- 

 Villee, C. A., Lowens, M., Gorden, M., Leonard, E. and Rich, A. (1949). The 

 incorporation of 32 P into the nucleoproteins and phosphoproteins of the develop- 

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Discussion 



Chairman: C. H. Waddington 



J. Brachet. The estimation of DNA in the cytoplasm of eggs depends largely on the 

 specificity of the method used. In the case of the sea-urchin and the frog, the more 

 specific the method, the less is the DNA found in the egg. Using Ceriotti's method, 

 which certainly is less specific than Dr. Hoff-Jorgensen's, I found about four times 

 as much DNA in frog eggs as he did. However, even so, a definite synthesis of 

 DNA was found to occur during cleavage. To explain these discrepancies other 

 methods will have to be tried. 



I am rather surprised at the conclusion that there is no DNA synthesis in chick 

 embryos until after the third day of incubation, as the embryos at that stage have 

 already undergone considerable morphogenesis. Is it possible that the yolk contains 

 substances interfering with Dr. Hoff-Jorgensen's method? The material giving a 

 positive Ceriotti reaction for desoxypentose is also in the yolk, but it is hard to see 

 how desoxyribonucleosides could be released from yolk platelets, without digestion of 

 the latter, in such a harmonious way that the liberated nucleosides would exactly 

 match the requirement for nuclear multiplication. If yolk constituents interfered 



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