Deoxynucleic acid in some gametes and embryos 



cells during the first stages of embryonic development. The egg of the sea-urchin 

 Paracentrotus lividus is much smaller than the egg of the frog and contains correspond- 

 ingly less DNA, namely only enough DNA for about 16 new diploid cells, in accord- 

 ance with the finding that the synthesis of DNA begins at that stage of development. 



The egg of the frog Rana platyrrhina contains about 10,000 times as much DNA as 

 the sperm and about 5,000 times as much DNA as the diploid cells. It seems reason- 

 able that the number of cells in the embryo at Shumway's stage 9, when DNA 

 synthesis begins, is a few thousands; Bragg (1938) found about 10,000 cells at 

 gastrulation in Bufo cognatus. The hen's egg contains enough DNA for about 5 X io 7 

 diploid cells, and synthesis of DNA begins after 3 days of incubation, indicating that 

 a 3-days old embryo contains about 5 x io 7 cells. This number of cells presupposes 

 an average doubling time of about three hours during the first three days of develop- 

 ment, as 5 x io 7 amounts to about 2 25 . 



Villee et al. (1949) found that inorganic 32 P is incorporated in the DNA of the sea- 

 urchin's egg even during the first hours after fertilization. This indicates that before 

 the increase in DNA begins the phosphate bonds of the cytoplasmic DNA are hydro- 

 lysed and the deoxynucleosides used in synthesis of specific nuclear DNA in the new 

 cells. Two types of conversions of one deoxynucleoside to another are known. One 

 is catalysed by a mammalian liver enzyme (Friedkin and Kalckar, 1950) : 

 Deoxyribose-i-R + H 3 P0 4 % Deoxyribose-i -phosphoric acid + R; Deoxyribose- 

 1 -phosphoric acid + Ri % Deoxyribose-i-Rj + H 3 P0 4 ; the other is catalysed by 

 extracts of some micro-organisms (McNutt, 1952) : 



Deoxyribose-i-R -fR^ Deoxyribose-i-Ri + R where R and R t represent purines 

 or pyrimidines. 



SUMMARY 



(1) A microbiological method, which allows the determination of a few fig. DNA is 

 described. 



(2) Using this method the following values for the DNA content of eggs and sperm 

 have been found. In Paracentrotus lividus: 16*6 x io~ 6 /xg./egg; 0*71 x io~ 6 

 /xg./sperm. In Rana platyrrhina: 7-3 x io~ 2 /xg./egg; 8-6 x io -6 ^g./sperm. In the 

 domestic fowl: 118 ju,g./egg. 



(3) In the same species the DNA content in the embryos has been followed during 

 development. An increase in the DNA content begins in the sea-urchin embryo at 

 the 16-cell stage, in the frog embryo at about the 5000-cell stage, and in the hen 

 embryo probably at about the 5 x io 7 -cell stage. 



The author is indebted to Statens Almindelige Videnskabsfond for a grant and to 

 Dr. R. Dohrn, Stazione Zoologica, Naples for help and hospitality. Grateful acknow- 

 ledgement is made for many helpful suggestions and interesting discussions to Dr. 

 E. Zeuthen of Copenhagen and Dr. A. Monroy of Palermo. Dr. Scherbaum of 

 Copenhagen kindly made the microscopic examinations of the eggs and sperm. 



87 



