OUTLINE OF INDIRECT DIVISION OR MITOSIS 51 



figure, consisting of the spindle and asters which, in general, stain 

 but slightly. The fibrous substance of the achromatic figure is gener- 

 ally known as archoplasm (Boveri, '88), but this term is not applied 



to the centrosome within the aster. 



f 



2. Metaphasc. The propJiascs of mitosis are, on the whole, pre- 

 paratory in character. The metaphase, which follows, forms the 

 initial phase of actual division. Each chromosome splits lengthwise 

 into two exactly similar halves, which afterwards diverge to opposite 

 poles of the spindle, and here each group of daughter-chromosomes 

 finally gives rise to a daughter-nucleus (Fig. 20). 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. 

 38, 39). Such variations do not, however, affect the essential fact 

 that tJie chromatic network is converted into a tJiread^- which, whether 

 continuous or discontinuous, splits tJiroughout its entire length into 

 two exactly 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 

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

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

 vi at in from the mother-nucleus. It is very important to observe that 

 the nuclear division always shows this exact equality, whether division 

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

 example (p. 131), 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 

 of the daughter-cells (cf. Figs. 43, 71) 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, 2 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 



1 It was this fact that led Flemming to employ the word " mitosis" (/U,TOS, a thread). 



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

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

 and appropriate term clearly has priority. 



3 Verbinduiigsfasern of German authors; filaments rennissants of Van Beneden. 



