xm] RESTING NUCLEUS 195 



observation of nuclei, however, especially in the telophase 

 and early prophase of mitosis, has provided evidence that 

 in some organisms the chromosomes, or at least the frame- 

 work or basis of them, may be traced continuously from one 

 mitosis to the next, and if this is true in some cases, the 

 failure to observe it in others does not prove that no con- 

 tinuity exists, but only that the conditions are unsuitable 

 for detecting it. One of the first instances of apparent con- 

 tinuity to be observed was described by BOVERI in the early 

 segmentation stages of the egg of Ascaris. In A. megalo- 

 cephala bivalens there are four long chromosomes with 

 thickened ends in the equatorial plate of the first segmenta- 

 tion division. They divide longitudinally, so that four 

 similar chromosomes travel to each pole and are included 

 in the nuclei of the two-cell stage. Their arrangement in the 

 equatorial plate varies in different eggs, as is shown in Text- 

 fig. 24. When the daughter nuclei are formed, projections 

 of the nuclei membrane are seen, corresponding with the 

 projecting ends of the chromosomes, and although in the 

 resting stage no trace can be seen of the chromosomes as 

 such, when the next nuclear division approaches the chro- 

 mosomes can be seen forming in the nucleus with their ends 

 in the projections that have persisted since the preceding 

 telophase. The impression gained from a study of such 

 nuclei is that although the chromosomes are invisible in the 

 resting stage, they, or their frame-work, have been there 

 all the time, and that they become visible in the prophase 

 of one mitosis in the exact position in which they disappeared 

 in the telophase of the one before. 



In the plant Carex aquatilis STOUT has been able to ob- 

 serve the chromosomes continuously from one mitosis to the 

 next through the whole of the resting stage. The only period 

 in which they are not recognisable is in the spireme and 



132 



