MITOSIS 



chromatin in the same structural condition as was the chromatin of the 

 resting mother nucleus (/). From the granular condition of the chro- 

 matin of a resting nucleus to the formation of the equatorial plate 

 there is a progressive series of chromatin changes which carries the chro- 

 matin toward the plane of cell cleavage (A to D inclusive). The 

 splitting of the chromosomes at this plane is an intermediate phase. 

 Following this there is a regressive series of chromatin phases during 

 which the divided chromatin travels away from the plane of cell divi- 

 sion (F-G-H-I). The progressive phases we choose to call the pro- 

 phases; the intermediate the metaphase, and the regressive series the 

 anaphases. The chromatin during all these phases stains deeply ; for 

 which reason the figures presented by the chromatin in mitosis have 

 been called the chromatic figures of mitosis. 



In all mitoses there are other structures, much less constant in ap- 

 pearance in various species, which do not stain readily. These are called 

 the achromatic structures or figures of mitosis. They furnish the path 

 along which the series of chromatin changes are run, and perhaps they 

 are the dynamic structures that control and direct the chromatin move- 

 ments. The chief or most common of 

 these is the spindle (see following numer- 

 ous figures). Ordinarily the outline of 

 this structure is fusiform. The shape, 

 however, is not constant. It may be 

 cylindrical- to barrel-shaped. In most 

 cases the spindle is striated, showing 

 that it is composed of delicate fibrils 

 arranged longitudinally and converging 

 at the poles. In a typical spindle there 

 is a central bundle of fibrils, the spindle 

 fibrils, extending from pole to pole, and 

 a second set of fibrils which extend 

 from chromosomes to poles. These sur- 

 round the central fibrils and have re- 

 ceived the name of mantle fibrils (Fig. 

 21). The spindle always lies at right 

 angles to the plane along which the cell 

 divides, with its equator in this plane, 

 so that it is about the equator of the spindle that the equatorial plate of 

 chromosomes is formed. As the daughter chromosomes separate, a 

 plate of granules is formed, in many spindles, through the equator 

 (see Fig. 34). 



These granules appear, one on each fibril, and they unite to form 

 collectively a structure known as the cell-plate. The cell-plate spreads 



FIG. 21. Mitotic figure, spermatago- 

 nium of Podophyttum. Shows the 

 spindle fibrils reaching from pole to 

 pole and the thicker mantle fibrils 

 reaching from pole to ends of chromo- 

 somes. 



