CHAPTER XI 

 THE ACHROMATIC FIGURE 



The separation of the longitudinally divided chromosomes into two 

 daughter groups involves the action of the achromatic figure. In many 

 cells, notably those of vascular plants, this consists solely of the spindle, 

 a bipolar structure formed chiefly by achromatic constituents of the 

 nucleus. In other cases, including most animal cells and those of certain 

 lower plants, the figure has in addition at each pole a system of cytoplas- 

 mic radiations known as the aster, often with a centrosome at its focus. 

 Achromatic figures of the latter type are known as "amphiastral" 

 figures, while those without asters are called "anastral" figures. 



Anastral Figures. — All vascular plant cells, with the exception 

 of certain spermatogenous cells of species with motile spermatozoids, have 

 anastral achromatic figures. A typical example is seen in the root-tip 

 cell, where the development occurs in the following manner. 



According to an interpretation set forth in the preceding chapter, 

 the metabolic nucleus contains within its membrane three easily dis- 

 tinguishable components: a reticulum made up of anastomosed chromo- 

 nemata, a nucleolus (or nucleoli), and karyolymph. As the prophase of 

 mitosis advances, each chromonema becomes more or less free from its 

 neighbors and is seen to be surrounded by a distinct mass of matrix 

 substance. The karyolymph, in which the developing chromosomes lie, 

 is to form the spindle; hence it may be termed ''spindle substance."^ 

 In the root-tip cell (Fig. 81) the nucleus commonly enlarges during the 

 prophase. Its membrane then gradually shrinks inward from the two 

 poles, leaving behind two "polar caps" of spindle substance, which seems 

 to have filtered through the membrane. As the membrane shrinks more 

 closely about the crowded chromosomes, the caps become larger and 

 gradually form a more definitely spindle-shaped figure. Eventually the 

 membrane disappears and the double chromosomes become arranged 

 with their spindle-attachment regions in the equator of the spindle. 

 Through all these changes the appearance of the cytoplasm remains 

 essentially unchanged (Robyns, 1924, 1929). 



The spindle usually terminates in rather sharp points at its poles, 

 but cases are known in which the polar regions are nearly or quite as 

 wide as the equator. In the living condition the spindle is optically 



' It is called "parachromosomic substance" by Koerperich (1930) and the "para- 

 genoplast" by Bleier (1930c). 



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