Active and Inert Regions of the Chromosome 221 



During the resting stage which follows, the euchromatin has al- 

 most no nucleic acid. During the following cell division, the 

 amount of nucleic acid in the euchromatic regions begins to 

 increase at prophase and reaches the maximum again in the 

 metaphase and anaphase of this division. Although the hetero- 

 chromatin also contains its maximum amount of nucleic acid at 

 metaphase and anaphase, it does not lose so much during telo- 

 phase and has a fairly high content even during the resting stage. 

 Consequently, with nuclear stains, the chromosomes at meta- 

 phase and anaphase are stained uniformly deeply, whereas, dur- 

 ing the resting stage, the heterochromatin alone becomes stained. 

 Studies on Fritillaria pudica show that, during the resting stage, 

 the heterochromatic parts of the chromosomes not only obtain 

 all the nucleic acid and therefore become very deeply stained, 

 but they may also fuse to give branched structures. For that 

 reason, the number of deeply staining bodies in the resting 

 nucleus does not always correspond to the number of hetero- 

 chromatic parts of the chromosomes. 



Since both the euchromatin and heterochromatin contain large 

 amounts of nucleic acid when the chromosomes are in metaphase 

 and anaphase, these chromosomes are normally stained evenly 

 and appear to have a smooth outline. Darlington and La Cour, 

 however, have shown that the heterochromatic regions of Fritil- 

 laria pudica, Paris, and Trillium can be differentiated from the 

 euchromatic regions by keeping the plants at a temperature 

 below 3° centigrade and by allowing them to undergo mitosis 

 at that temperature. The low temperature reduces the amount 

 of nucleic acid in the nucleus. Normally, there is enough of this 

 material for all the parts of the chromosomes, but in the cold- 

 treated plants, the amount is reduced to such an extent that 

 there is no longer a sufficiency. During mitosis of plants raised 

 at these low temperatures, the reduced amount of available 

 nucleic acid is taken up by the euchromatin and, therefore, the 

 heterochromatin has less than it has normally. As a result, the 

 euchromatic regions of the mitotic chromosomes during meta- 

 phase and anaphase become stained very deeply, whereas the 

 heterochromatic regions are stained less deeply and seem thinner, 

 appearing as slight constrictions (Fig. 69). Apparently there is 

 considerable variability in the nucleic acid content of the hetero- 



