CELL-DIVISION AND NUCLEAR DIVISION 59 



attach themselves to the chromosomes and "supporting-fibres" which 

 connect the poles with one another and, by furnishing the necessary 

 resistance, enable the traction-fibres to pull the chromosome-halves 

 apart. Another group of investigators regards the appearance of spindle- 

 fibres merely as a consequence of special physical and chemical changes, 

 originating mainly in the centrosomes which are of such regular 

 occurrence in animal cells ; according to this view the spindle-fibres 

 are, as it were, comparable to "lines of magnetic force," and at most 

 represent the paths along which the chromosomes travel. It is reserved 

 for future workers in this field to decide between these divergent 

 opinions. 



The " supporting " spindle-fibres persist for some time after the 

 reorganisation of the daughter-nuclei is completed in the form of the 

 so-called "interzonal fibres" or " connecting fibres" {Vcrbindungsfadcn). 

 Later these become reinforced by the interpolation of a number of 

 additional fibrils, and as a consequence the whole mitotic figure 

 expands and becomes more barrel-shaped (Fig. 7 E). If the dividing 

 cell is rich in protoplasm, and not too wide, the expanding spindle finally 

 comes into contact with the lateral walls in the plane in which the 

 new transverse septum will later be laid down. The fibres then 

 become thickened in the same plane, and the rod-like bodies thus pro- 

 duced fuse to form the so-called cell-plate (Fig. 7 e) ; this structure is 

 at first a homogeneous plasmatic membrane similar in character to the 

 ectoplast, but subsequently splits into two layers, between which the 

 new cell-wall is secreted (Fig. 7 f). In this way the mother-cell forms 

 two daughter-cells by simultaneous segmentation. If the cell has a 

 large sap-cavity, and is consequently so wide that the expanding 

 mitotic figure does not touch the lateral walls all round, the spindle 

 moves across gradually from one side of the cell to the other, and in 

 so doing lays down the new transverse wall successively. As soon as 

 the division of the mother-cell is completed by the formation of the 

 new cell-wall the interzonal fibres gradually lose their distinctness, 

 and finally become merged in the cytoplasm of the daughter-cells. 



Among Thallophyta the connection between nuclear and cell- 

 division is not so intimate as in the case of Higher Plants ; the new 

 cell-walls are not formed within bundles of connecting fibres. Where 

 the cells are uninucleate, the nucleus first divides independently. A 

 new wall then arises, either simultaneously at the expense of a 

 previously differentiated cell-plate, or else successively. In the latter 

 case it begins as an annular ridge, laid down all round upon the lateral 

 wall ; the ridge extends further and further inwards until it finally 

 cuts the protoplast in two and closes up to form a complete septum. 

 Even in such cases, however, cell-division appears to stand in a 



