70 



CELL-DIVISIOX 



finally gives rise to a daiii^^hter-nucleus (Fig. 26). In some cases 

 the splitting of the chromosomes cannot be seen until they have 

 grouped themselves in the equatt)rial plane of the spindle ; and it is 

 only in this case that the term " metaphase " can be applied to the 

 mitotic figure as a whole. In a large number of cases, however, the 

 splitting may take place at an earlier period in the spireme-stage, or 

 even, in a few cases, in the reticulum of the mother-nucleus (Figs. 

 54. 55). Such variations do not, however, affect the essential fact 

 that the clnvDiatic nctivork is converted into a thread^ which, ivhether 

 continuous or discontinuous, splits tJiroughojit its entire lengtJi into 

 two exactlj equivalent halves. The splitting of the chromosomes, 

 discovered by Flemming in 1880, is the most significant and funda- 

 mental operation' of cell-division; for by it, as Roux first pointed out 

 {^^}i\ the entire substance of the chromatic network is precisely halved, 

 and the daughter-nuclei receive precisely equivalent portions oj chro- 

 matin from the mother-nucleus. It is very important to observe that 

 the nuclear division always shows this exact quality, whether division 

 of the cell-body be equal or unequal. The minute polar body, for 

 example (p. 238), receives exactly the same amount of chromatin as 

 the Q^^g, though the latter is of gigantic size as compared with the 

 former. On the other hand, the size of the asters varies with that 

 of the daughter-cells (Figs. 58, 175), though not in strict ratio. The 

 fact is one of great significance for the general theory of mitosis, 

 as will appear beyond. 



3. Anaphases. — After splitting of the chromosomes, the daughter- 

 chromosomes, arranged in two corresponding groups,^ diverge to oppo- 

 site poles of the spindle, where they become closely crowded in a mass 

 near the centre of the aster. As they diverge, the two groups of 

 daughter-chromosomes are connected by a bundle of achromatic 

 fibres, stretching across the interval between them, and known as the 

 interzonal fibres or connecting fibres? In some cases these differ in a 

 marked degree from the other spindle-fibres ; and they are believed 

 by many observers to have an entirely different origin and function. 

 A view now widely held is that of Hermann, who regards these fibres 

 as belonging to a central spindle, surrounded by a peripheral layer 

 of mantle-fibres to which the chromosomes are attached, and only 

 exposed to view as the chromosomes separate.^ Almost invariably 

 in the division of plant-cells and often in that of animal cells these 



1 It was this fact that led Flemming to employ the word mitosis (fiiros, a thread). 



2 This stage is termed by Flemming the dyasier, a term which should, however, be aban- 

 doned in order to avoid confusion with the earlier word amphiafter. The latter convenient 

 and appropriate term clearly has priority. 



3 Verlnndutigsfasern of German authors ; filaments reunissants of Van Beneden. 



* Cf. p. 105. 



