THE MITOTIC CYCLE 



fixative used, though his claim that the living nucleoplasm is homo- 

 geneous needs to be examined by further investigations with the use of 

 modern optical methods, 



Chromonemata — The visibility of the chromonemata in the living 

 intermitotic plant nucleus is variable, and can be altered by treatment 

 of the cell with media of differing tonicity (Shinke^i). Both the osmotic 

 pressure of the guard cells of stomata (Wiggans^^) and the appearance 

 of their nuclei (Weber^^) alter when these structures open and close. 

 The differences in refractive index between chromonemata and nuclear 

 sap are thus influenced by the water content of the whole nucleus. 



The chromosomes of the interphase nucleus, as we have said, are 

 represented by two components, the heterochromatic granules, which 

 are segments of the chromosomes which persist apparently unchanged, 

 together with the 'backbone' threads, the chromonemata. Animal 

 nuclei, in which the latter can be demonstrated are regarded by Serra^* 

 as characteristic of secretory cells and of tissues in which frequent cell 

 division is occurring. He finds that in an immature snail oocyte, before 

 meiotic prophase begins, chromonemata are not so readily visible. It 

 would be of great interest to extend such studies on interphase nuclei 

 from this point of view. Shiwago^^ was able to demonstrate slowly 

 moving threads within intermitotic leucocytes of the frog, visible in a 

 bright-field microscope. 



The chromonema is the residual part of most of the chromosome 

 after the greater part of its nucleoproteins have been dispersed in 

 telophase. The first stage in the process is that the chromosomes in late 

 anaphase swell into vesicles. This can most readily be seen where 

 one or two chromosomes lie apart from the main daughter group. 

 The early telophase nucleus can thus be regarded as a mass of 

 chromosomal vesicles. The nuclear membrane is formed from those 

 sectors of their walls which form the interface between cytoplasm 

 and nucleus. Elsewhere, their adjoining walls do not persist intact, 

 but in the embryo of the fish Fundulus (RichardsI^) the vesicles 

 are separately distinguishable throughout interphase (Figure 9), 

 Accarding to Kater, in resting nuclei both of the frog^' and of the 

 rat^^ the intervesicular boundaries can still be traced though it must be 

 admitted that the author's illustrations of this are not entirely con- 

 vincing. Lewis^^ supports the view that the interphase nucleus is 

 morphologically vesicular. 



It is clear that as the chromosomes swell in telophase chromatinic 

 material is detached from their 'backbone' threads. Ris and Mirsky^" 

 speak of the chromosomes as then being in the 'extended state', though 

 these authors overlook the persistence of the chromonemata in some 

 nuclei, and consider that the deoxynucleoproteins of the chromosomes 

 are evenly spread throughout the living interphase nucleus. The usual 



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