THE MITOTIC CYCLE 



The name 'heteropycnosis' was suggested by Gutherz^"^ for this 

 general type of behaviour. It was first shown to apply to segments of 

 the autosomes by Heitz^^-' in 1928, who studied resting and dividing 

 nuclei in a number of liverworts and mosses. Those 'heterochromatic' 

 parts of the chromosomes which persist unchanged during telophase 

 can be traced into the resting nucleus where mostly they are associated 

 with the nucleolus. Such bodies in the interphase nucleus had long been 

 recognized; they were called 'chromocentres' by Baccarini.^"^ In plants 

 their distinction in staining reaction from the nucleoli was recognized 

 by RosEN^"^ in 1892. 



Figure 1 3 Second spermatocytes of Mecostethus grossus in interphase a 

 without X- b with X-chromosome. The heteropycnotic autosome 

 (VII) is visible in both. From White'^"'^ {By courtesy, Proc. roy. Soc. B). 



This condition in which part or all of a chromosome is still loaded 

 with chromatin when elsewhere in the 'euchromatic' regions the charge 

 has been partly lost is now known as positive heteropycnosis. Examples 

 of the contrary negative condition are known, and have been described 

 by White^^" in the eafly spermatogonial divisions in the short-horned 

 grasshoppers and crickets, where at metaphase the X-chromosome is 

 less charged than are the autosomes (Figure 13). The reverse is true at 

 diakinesis. Thus this example may be looked on as a difference in phase 

 during the cycle of the migration of chromatin to and from the two 

 types of chromosome. This aspect of heteropycnosis has been termed 

 'allocycly' by Darlington and La Cour.^^^ These authors find that 

 negatively heterochromatic segments of the metaphase chromosomes in 

 Paris and Trillium are found in plants kept at 0° C. and claim that the 

 chromocentres of the resting nuclei represent the same chromosomal 

 regions as those in which the phenomenon of 'nucleic acid starvation' 

 is found at low temperatures. Similar observations on animal cells have 

 been made by Callan^^^ in the newt. In the spermatocytes of the toad, 



44 



