3 

 THE INTERPHASE NUCLEUS 



NUCLEUS OF CHICK AND MAMMALIAN CELLS IN LIVING TISSUE CULTURES 



The interphase nucleus is bounded by the nuclear membrane which is 

 sharply outlined in the living cell. Within, the most obvious structures 

 are the nucleoli, either one or two in cells of the chick and more 

 numerous in those of mammals. Scattered at intervals throughout the 

 nucleus are a larger number of smaller bodies, which are sometimes 

 referred to as the 'chromocentres'. They differ in composition from the 

 nucleoli, for whereas the latter mainly consist of ribonucleoproteins, 

 the chromocentres are made of deoxyribonucleoproteins, and so are 

 Feulgen-positive (Plate II (4)). They are small segments of the chromo- 

 somes which persist apparently unchanged into interphase; and are 

 termed 'heterochromatic' (p 43) ; their surface is apparently adhesive, 

 for as Vanderlyn^ has observed, they stick either to the nuclear mem- 

 brane, or to the surfaces of the nucleoli, or to each other in aggregates. 

 The non-heterochromatic parts of the chromosomes in this type of 

 nucleus are represented by extremely fine threads, the chromonemata, of 

 which there may be more than one to each chromosome. In well fixed 

 nuclei, a uniform groundwork of chromonemata can be seen throughout 

 the nucleus, and some of these threads can occasionally be seen in life by 

 phase-contrast (Plate XII (18)). It is probable that the chromonemata 

 line the inside of the nuclear membrane, because in prophase when they 

 become more visible, they are then seen in this position. In time-lapse 

 films of cells in interphase, no movement within the nucleus can be 

 seen, whereas in the cytoplasm there is ceaseless activity. The nucleus 

 as a whole is a structure of some rigidity, though evidence from micro- 

 dissection (Chambers and Fell^) suggests that the nuclear sap is a 

 fluid. 



In succeeding pages, each of these nuclear components will be 

 further discussed. 



CHROMOSOMES AND THE DEOXYNUCLEOPROTEINS 



During the prophase of mitosis, chromosomes are formed within the 

 nucleus identical in number with those which were present in the 

 daughter group during the later phases of the preceding division. Dur- 

 ing the intervening period, the chromosomes are not usually recog- 

 nizable as such, and it is clearly a question of much importance from 



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