174 R. VENDRELY 



tids which will be derived from it; on the other hand, Alferf^ has found in 

 primary oocytes before meiosis four times as much DNA as in the pronuclei 

 resulting from the two meiotic divisions. 



In an investigation on mitosis, Swift^^ studied the behavior of DNA in 

 the nuclei of developing liver tissue of the 11 -day mouse embryo and in the 

 pronephros and erythrocyte nuclei of the recently hatched Amblystoma 

 larva. He noted for interphase nuclei DNA values spreading between the 

 normal diploid value and twice this amount. The posttelophase nuclei pre- 

 sented diploid values, whereas the early prophase nuclei had twice the 

 diploid value. The synthesis of DNA must therefore occur during inter- 

 phase before the visible stages of mitosis. Seshachar^^ found a similar pro- 

 cess in the division of the ciliate micronucleus. Alfert,^^ studying the early 

 development of mouse embryo, found that the DNA content of the nuclei 

 is always doubled prior to the onset of nuclear division. 



Finally, Walker and Yates^* have studied by ultraviolet absorption and 

 by the Feulgen method the DNA content of nuclei in tissue cultures . These 

 authors were able to determine the exact phase of the cell under considera- 

 tion by studying its history in a phase contrast film taken previously during 

 all the developmental stages. They found that posttelophase nuclei con- 

 tain the same amount of DNA as erythrocyte nuclei, i.e., the diploid 

 amount. This amount is doubled during interphase and reaches its maxi- 

 mum before prophase. This study upon living cells, the precise stage of 

 development of which can be accurately determined, seems to be more 

 reliable than results from fixed and stained material. It therefore seems well 

 established that the synthesis of DNA occurs during late interphase, before 

 prophase. Nevertheless, Pasteels and Lison,^^ working upon erythroblasts 

 of the rat embryo, Lieberkiihn glands of the adult rat, and fibroblasts of 

 the chick heart embryo in tissue culture have found that in these rapidly 

 growing cells the DNA content of the nucleus does not change during pro- 

 pihase. At anaphase the two daughter nuclei contain half this value, but 

 the initial value is reached again at telophase, so that the synthesis of the 

 DNA would appear to occur in telophase and would be completed during 

 the reconstruction of the daughter nuclei. These results are in complete 

 disagreement with the conclusions of other authors. In Fig. ] are summa- 

 rized the different theories for the synthesis of DNA. Further work is neces- 

 sary to clear up the discrepancies, but it should be pointed out, as Walker 

 and Yates^^ have done, that Pasteels and Lison compared spherical inter- 

 phase nuclei and the irregular filamentous area of the chromosomes. Errors 

 in photometric measurements are unavoidable in such cases, and Pasteels 

 and Lison themselves stress the great difficulty of such work. The concep- 



«' B. R. Seshachar, Nature 165, 848 (1950). 



92 J. Pasteels and L. Lison, Arch. biol. (Liege) 61, 445 (1950). 



