THE MITOTIC CYCLE 



threads may be seen within it, much smaller in diameter than the 

 anaphase chromosomes (Plates IX, X(i3) ). The swelling of individual 

 chromosomes may occasionally be seen in those separated by chance 

 from the main daughter group. At the interface between the cytoplasm 

 and the outer chromosomal vesicles, the nuclear membrane is formed 

 and expands in area, and the daughter nucleus continues to enlarge. 

 The structure of the nucleus at this stage is reticular, and the main 

 chromosome threads which are still Feulgen-positive, may be seen to be 

 double. Nucleoli seem to form wherever there are dense groups of 

 chromonemata and their relationship to individual chromosomes seems 

 here to be obscure from the first (p 42). 



At this stage, some thirty minutes after the beginning of anaphase, 

 the general structure of the nucleus consists of a coarse meshwork of 

 thick chromosome threads linking up the nucleoli. Gradually, these 

 threads differentiate into heterochromatic granules and fine chromo- 

 nemata, and the nucleoli undergo repeated re-arrangements in form 

 and position. The nucleus is still rapidly enlarging, and some three 

 hours after the beginning of anaphase is about two-thirds the size of 

 the mother nucleus in prophase. In full interphase, the structure of the 

 nucleus can no longer be called reticular. Occasional threads can still 

 be seen in the living state, but well-fixed material shows that the whole 

 nucleus is then uniformly filled with fine chromonemata. Alfert^^ 

 finds that there is no decrease in the relative DNA content of embryonic 

 mouse nuclei from telophase to full interphase. At this time the apparent 

 decrease in the intensity of Feulgen reaction as judged subjectively must 

 be due to dispersal of DNA throughout an increasing nuclear area.* 



In the foregoing descriptions of the nuclear changes in mitosis, the 

 swelling of the young daughter nuclei in telophase has been ascribed 

 to the absorption of water. Another explanation of telophase swelling 

 is given by Caspersson,^* mainly with reference to Grasshopper 

 spermatocytes. He claims that the chromosomes in telophase swell by 

 the accumulation of protein within them. 'During the course of the 

 swelling the chromosomes gradually fill the spaces between them, and 

 the rounded cell results.' In prophase the process is reversed and the 

 major part of the nuclear proteins disappears from the nucleus. The 

 marked volume changes of mammalian cells during mitosis are shown 

 by the data of Biesele et alii^^ here reproduced as Figure 26. 



Here clearly is a point of some importance which needs to be 

 decided; it should not be impossible to distinguish between increase 

 of protein and of water in the nuclei during telophase. The X-ray 

 method of Engstrom^^ for measuring the mass of cell constituents 



* Liso.N and Pasteels" maintain that the early post-telophase growth of the nucleus is 

 the period when DNA is synthesized in embryonic and adult rat tissues, in chick cultures, 

 and in Echinoderm larvae. 



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