146 Alma Howard and S. R. Pelc 



present in nuclei. Seshachar (1950) observed that in the 

 micronucleus of Chilodonella uncinatus the DNA content of 

 daughter nuclei soon after division was about one half that 

 of interphase nuclei. This would mean that the synthesis 

 of DNA Avas already complete before prophase began. Swift 

 (1950a) examined the pronephros of Ambystoma larvae, 

 where there is considerable mitotic activity, and found that 

 the earliest recognizable prophase stages had twice as much 

 DNA as most of the resting cells. He concluded that DNA 

 is built up in the interphase, and that prophase is initiated 

 when the doubled amount has been reached. 



In plant tissues having a high mitotic rate, Swift (19506) 

 found essentially the same picture. Early prophases had 

 twice the telophase amount of DNA, and the interphase 

 values fell between the two. In the root of Zea mays most 

 interphase nuclei showed the double amount, a fact which 

 suggests that synthesis takes place soon after telophase and 

 that there may be a lag between the end of synthesis and the 

 beginning of prophase. 



Lison and Pasteels (1951) have recently put forward 

 evidence that in the segmenting egg of Paracentrotus lividus 

 synthesis of DNA occurs during the reconstruction of the 

 daughter nuclei at telophase. In an earlier paper (1950) they 

 had concluded that in certain vertebrate embryonic and adult 

 tissues the DNA content of the daughter nuclei is doubled 

 between anaphase and the end of telophase, and they suggest 

 that this observation may have general validity. 



From the observations described here and summarized 

 in Fig. 2, it is plain that in the meristem of Vicia faha the 

 synthesis of DNA occurs during interphase, and that a time 

 interval of about 6 hours exists between the end of svnthesis 

 and the beginning of visible prophase. It follows that the 

 morphological appearance of chromosomes is not an imme- 

 diate expression of the synthesis of new DNA molecules, 

 although the arrangement of these molecules may alter as the 

 chromosomes become visible. In considering the significance 

 of the conclusions of Seshachar, Swift and ourselves, it should 



