70 



CELL-DIVISION 



finally g-ives rise to a dau.t^hter-nucleus ( Mg. 26). In some cases 

 the splitting; of the chromosomes cannot be seen until they have 

 grouped themselves in the equatorial 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 chrouiatic nctivork is converted into a tJiread'^ zuhich, wJicther 

 continuous or discontinuous, splits tJiroughout its entire length into 

 tivo exactly equivalent halves. The splitting of the chromosomes, 

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

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

 ('83), the entire substance of the chromatic network is precisely halved, 

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

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

 the nuclear division always shows this exact quaHty, 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 egg, though the latter is of gigantic size as compared with the 

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

 .jof 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 7nantle-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 {fxiroi, a thread). 



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

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

 and appropriate term clearly has priority. 



^ Verhindtiiigsfasern of Cierman authors ; fdainents rcunissants of Van Beneden. 



4 Cf. p. 105. 



