THE MITOTIC CYCLE 



of the chromosomes during cell division are due to Caspersson.^'^ By 

 means of ultraviolet microspectrophotometry, he was able to calculate 

 the total nucleic acid content of the nuclei of the spermatocytes of the 

 grasshopper Gomphocerus between the leptotene and diplotene stages. 

 Some increase during early leptotene was found, but thereafter the 

 amount of nucleic acid present remained approximately constant. This 

 point was not confirmed by the later work of Ris,^'i whose extinction 

 measurements at 2,53 7 A on spermatocyte nuclei of Chortophaga indi- 

 cated a considerable increase in total nucleic acids between the lepto- 

 tene and pachytene stages; in onion root-tip nuclei such estimations 

 between prophase and metaphase showed a similar increase of approxi- 

 mately twofold. In interphase and prophase, the total nucleic acid 

 content of the onion nucleus appeared to be the same. Ris made other 

 measurements at appropriate wavelengths on material stained by 

 several cytochemical methods. In Feulgen preparation of onion roots, 



16 

 Unils of DNA 



Figure 36 Distribution of DNA content in arbitrary units of nuclei 

 of larval Amhystoma. Solid lines: interphase nuclei of liver of two size 

 classes. Dotted lines: pronephric nuclei in telophase (left'), and in pro- 

 phase (right). From Swift^'* (By courtesy, Physiol. ^ooL). 



dividing cells showed an increase in DNA content both on entering pro- 

 phase and also between this period and metaphase. In similar measure- 

 ments on embryonic mammalian and larval amphibian tissues, Swift^'^ 

 finds that the range of relative DNA content in interphase nuclei is 

 approximately twofold, and that the values for all nuclei in prophase are 

 at the upper end of this range (Figure 36) . His conclusion is that the 

 DNA which will be required for the next division is synthesized during 

 interphase, and once the cell has entered mitosis, there is no further in- 

 crease in Feulgen-positive material.* Thus Swift and Ris, using diflferent 

 material, do not agree on this last point. The degree of accuracy of 

 these microcolorimetric measurements is uncertain. Ris's figures show 

 only a very approximate halving of the total DNA content of the 



* This conclusion is confirmed by similar measurements made by Alfert^'^ on cleavage 

 stages in the mouse. By using bulk methods of analysis, Ogur et alii^''* have shown that the 

 DNA content of the microspores of Lilium increases linearly in the interphase between 

 meiosis and the succeeding mitosis. In microphotometric measurements, again on Feulgen- 

 stained material, Lison and Pasteels^'** have found that DNA is rapidly synthesized in or 

 after telophase ('des I'apparition de la membrane nucleaire'), both in embryonic and adult 

 rat tissues, in chick cultures, and in the eggs and larvae oi Paracentrotus lividus. 



102 



