THE INTERPHASE NUCLEUS 



Bufo arenarum, Saez et alii^^^ find that one autosome in metaphase I 

 normally is negatively heteropycnotic (Figure 14). 



Such are perhaps the main facts concerning this question which 

 descriptive cytology has so far provided, although Schultz^^* is of 

 opinion that all the various features of heterochromatin in different 

 organisms when added together 'come pretty close to including all the 

 possible aspects of chromosome behaviour'. To mould this subject into 

 a presentable form, it has been found necessary to mix in a large 

 proportion of theoretical matrix, within which it is not always easy to 

 discern other ingredients. It is believed that there are special features 

 of heterochromatin in its genetical properties (Schultz^^*), in its 

 high susceptibility to breakage by mutagenic chemicals (Loveless and 



Figure 14 Negative heteropycnosis of an 



autosome in metaphase I of meiosis in 



the toad Bufo arenarum. After Saez et alii^^^ 



{By courtesy, Z- /^^//.f/owcA.) . 



Revell^^^), and also in its chemical composition. Some of these 

 generalizations are based on observations in Drosophila; in the larval 

 salivary gland nucleus next to the nucleolus there is a single chromo- 

 centre made up of all the heterochromatic regions near the centromeres 

 (Figure 15), it includes the whole of the Y-chromosome of the male. 

 Caspersson^i^ concluded from a comparison of the ultraviolet absorp- 

 tion spectra of this and the adjacent structures in the salivary nucleus, 

 that basic proteins are characteristic of the heterochromatic regions. 

 There is, however, some doubt whether nucleoproteins can be charac- 

 terized by this method alone (p 12), In the nuclei of the spinal 

 ganglion cells of the rabbit, Hyden^^'^ has shown that both the nucleolus 

 and the chromocentre have a strong affinity for acidic dyes. It is not yet 

 certain, however, whether the euchromatic regions at metaphase differ 

 in this respect. 



In Drosophila, variation of the total amount and arrangement of the 

 heterochromatin have been shown to be correlated with changes else- 

 where within the cell. Caspersson and Schultz^^^ showed that where 

 a Y-chromosome was included in the oocyte nucleus, cytoplasmic 



45 



