CHAPTKR IX 



THE ACHROMATIC FIGURE, CYTOKINESIS, AND THE CELL 



WALL 



THE ACHROMATIC FIGURE 



The spindle fibers and asters about the centrosomes (when these are 

 present) are collectively termed the achromatic figure, in contradistinction 

 to the chromatic figure, or chromosomes. Compared with the chromo- 

 somes the achromatic figure is relatively little understood, which makes 

 it a very unsatisfactory subject for discussion. We shall first describe 

 the achromatic figure in its more common forms, and after mentioning 

 certain theories which have been propounded to explain its origin and 

 nature we shall briefly review a few of the suggestions which have been 

 made on the subject of the mechanism of mitosis. 



In Higher Plants. — In somatic mitosis in higher plants the achromatic 

 figure is devoid of centrosomes and asters. Ordinarily it arises and be- 

 haves as follows: While the prophasic changes are taking place within 

 the nucleus the first indications of spindle formation appear in the cyto- 

 plasm in the immediate vicinity of the nucleus. At the two sides of the 

 latter, in the general position of the future spindle poles, there arc de- 

 veloped two masses of more or less hyaline material, usually called "kino- 

 plasmic caps." In these two polar caps delicate fibrils soon appear, as 

 if by a process of condensation (Fig. 58, A, B). The nucleus commonly 

 shrinks at this time, while the fibrous areas increase in size and together 

 form a more definitely spindle-shaped figure. After the nuclear mem- 

 brane has shrunken more closely about the chromosomes it goes into 

 solution and the ingrowing fibers attach themselves to the longitudinally 

 split chromosomes. In many cases the membrane disappears without 

 shrinking, the fibers growing considerably in length to reach the chromo- 

 somes. The latter quickly become regularly arranged in the equatorial 

 plane preparatory to their separation (Chapter VIII). The mitotic 

 figure is now established (Fig. 48). The many fibers composing the 

 spindle may focus at a single sharp point at each pole, or they may end 

 indefinitely without converging to a point, forming in the hitter case a 

 broad-poled figure which in extreme cases may be as wide at the poles 

 as at the equator (Fig. 74, D). Some of the fibers extend from the poles 

 to the chromosomes, to which they are attached, while others pass 

 through from otic pole to the other without being so attached: these 



17.-. 



