62 BOTANY 



the longitudinal halves of the chromosomes are drawn in opposite 

 directions towards either pole of the division figure (5, 6, 7). In the 

 process of this movement of the chromosomes towards the poles, the 

 other continuous spindle fibres seem to serve as supports. Before the 

 chromosomes, however, reach the poles, a division of the centrospheres 

 (5), commencing with their centrosomes, takes place, so that two 

 centrospheres are previously provided for each new daughter nucleus. 



In the nuclei of vegetable cells, the primary spindle fibres connected with the 

 chromosomes unite with the spindle fibres extending from pole to pole. The number 

 of these secondary spindle fibres corresponds with the number of the chromosomes. 



In forming the daughter nuclei, the free ends of the chromosomes 

 first become drawn in (8), and the surrounding cytoplasm separates 

 itself by means of a protoplasmic membrane (9) from the developing 

 nuclei. Within the nuclear cavities which are thus produced the 

 chromosomes elongate (10), and joining together, end to end, become 

 again intertangled. The chromatin substance is diminished in quantity, 

 nucleoli at length appear in the enlarging nuclei, and finally a con- 

 dition of rest is again reached. 



The changes occurring in a mother nucleus preparatory to division are termed 

 the pkophases of the karyokinesis. These changes extend to the formation of the 

 nuclear plate, and include also the process of the longitudinal division of the 

 chromosomes. The separation of the daughter chromosomes is accomplished in the 

 metaphases, and the formation of the daughter nuclei in the anaphases of the 

 karyokinesis. The real purpose of the whole process is consummated in the 

 quantitative and qualitative division of the chromosomes, resulting from their 

 longitudinal segmentation (4, B, 0). The anaphases of the karyokinesis are but a 

 reverse repetition of the prophases. Exceptions to the process as here described, 

 are not of special importance, and need not be discussed. 



In ; addition to the mitotic or indirect nuclear division there is 

 also a direct or amitotic division, sometimes called Fragmenta- 

 tion (Fig. 63). It usually occurs in old cells, or in cells in which the 

 cell contents become disorganised shortly after the nuclear division. 



Instructive examples of direct nuclear division are afforded by the 

 long internodal cells of the Stone worts (Characeae), and also by the 

 old internodal cells of Tradescantia (Fig. 63). 



The direct nuclear division is chiefly a process of constriction 

 which, however, need not result in new nuclei of equal size. In the 

 case of the Stoneworts, after a remarkable increase in the size of the 

 nucleus, several successive rapid divisions take place, so that a con- 

 tinuous row of bead-like nuclei results. The old internodal cells of 

 Tradescantia (Fig. 63) very frequently show half-constricted nuclei of 

 irregular shape. While in uninuclear cells indirect nuclear division is 

 as a rule, followed by cell division, this is not the case after direct 

 nuclear division. 



