MODIFICATIONS OF MITOSIS 



59 



The variations of the chromatic figure must for the most part be 

 considered in the more special parts of this work. There seems 

 to be doubt that a single continuous spireme-thread may be formed 

 (cf. p. 184)-, but it is equally certain that the thread may appear from 

 the beginning in a number of distinct segments ; i.e. as a segmented 

 spireme. The chromosomes, when fully formed, vary greatly in 

 appearance. In many of the tissues of adult plants and animals 



Fig. 25. Division of pollen-mother-cells in the lily. [GUIGNARD.] 



A. Anaphase of the first division, showing the twelve daughter-chromosomes on each side, the 

 interzonal fibres stretching between them, and the centrosomes, already double, at the spindle- 

 poles. B. Later stage, showing the cell-plate at the equator of the spindle and the daughter- 

 spiremes (dispireme stage of Flemming). C. Division completed; double centrosomes in the 

 resting cell. D. Ensuing division in progress ; the upper cell at the close of the prophases, the 

 chromosomes and centrosomes still undivided ; lower cell in the late anaphase, cell-plate not yet 

 formed. 



they are rod-shaped and are often bent in the middle like a V (Figs. 

 21, 33). They often have this form, too, in embryonic cells, as in 

 the segmentation-stages of the egg in Ascaris (Fig. 24) and other 

 forms. The rods may, however, be short and straight (segmenting 

 eggs of echinoderms, etc.), and may be reduced to spheres, as in 

 the maturation stages of the germ-cells. 



