Deoxynucleic acid in some gametes arid embryos 



with Dr. HofF-Jorgensen's method, this would explain the much greater quantity of 

 DNA in a hen's egg than in a sea-urchin egg. It would be interesting to study separ- 

 ately the DNA content of the embryos and of the yolk, and also to study the DNA 

 content of embryos grown on Spratt's synthetic medium. 



E. Hqff-Jergensen. It is unlikely that other substances are responsible for the growth 

 of the test organism. The exact amount of desoxyriboside found by the assay method 

 can be recovered as DNA from the cells of the test organism. Also the hypothetical 

 substance supposed to interfere with the test would have to be formed as a result of 

 treatment with the highly specific crystalline DNA-ase; little or no growth factor 

 is found without this treatment. 



C. H. Waddington. It seems peculiar that the egg should not perform any synthesis of 

 DNA until all its reserves are completely exhausted, and should then immediately 

 start to synthesize at full speed. Moreover, at the time when synthesis begins in the 

 chick embryo, the centre of the yolk is still a long distance away from the nearest 

 cells (those of the yolk-sac). Is it possible that there is a continuous destruction of 

 DNA, and that the synthesis at first balances this but that, eventually, as the number 

 of nuclei increases, synthesis greatly surpasses breakdown ? 



E. Hoff-Jergensen. We are determining the linkage between desoxyribose and a base, 

 and all we can say is that during the first three days of development the number of 

 these linkages is constant. 



J. E. Harris. If one is prepared to admit that the bacterium cannot synthesize DNA, 

 it is not unreasonable to suggest that the early embryo may not be able to do so 

 either. 



0. Maalee. A breakdown of DNA to balance synthesis would have to be taken all the 

 way down — beyond the nucleoside stage — if the degradation products are not to be 

 detected by Dr. Hoff-Jorgensen's method. 



C. H. Waddington. It would be interesting to try diploid or tetraploid frog embryos, 

 as these would be likely to contain a different amount of DNA per cell. 



J. Bracket. I have investigated haploid frog embryos produced by irradiation of the 

 sperm with ultra-violet light. These embryos contain more cells than diploid embryos. 

 There is less DNA in early cleavage stages; the DNA content catches up by the late 

 blastula stage; but after gastrulation the haploid embryo lags behind once more in 

 DNA content. 



M. M. Swann. Early embryos relying on a store of DNA might be immune to in- 

 hibitors of DNA synthesis, such as aminopterin. Have any been tried ? 



E. Hoff-Jergensen. The inhibitors tried — acting against thiamine, riboflavin, or folic 

 acid — had no visible effect on the embryos. It is of course possible they they do not 

 penetrate the egg membrane. 



E. jV". Willmer. Embryo extract stimulates the synthesis of nucleoproteins in tissue 

 cultures of chick fibroblasts. One of the immediate effects of the addition of embryo 

 extract is to increase the glucose uptake by the cells. It is therefore interesting to 

 observe, as I did many years ago, that the sea-urchin egg does not pick up glucose 



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