THE ORGANIZATION OF THE INDIVIDUAL 19 



The sister chromatids remain attached and closely parallel throughout 

 the prophase, but each chromatid becomes progressively shorter and 

 thicker both by coiling into a tight spiral and by actual growth. During 

 this process the several chromosomes appear to repel one another and 

 come to take positions as far apart as possible within the periphery of the 

 nuclear membrane, with their centromeres in the region of the equatorial 

 plane of the nucleus. 



Meanwhile other changes have been taking place in the nucleus. The 

 nucleolus has disappeared (in some kinds of cells the nucleolus or nucleoli 

 may be seen to be attached to one or more of the chromosomes). The 

 centrosome has become more conspicuous, and the centrioles have moved 

 apart along the surface of the nuclear membrane. The centrioles continue 

 to move farther apart, until they lie at opposite poles of the nucleus; they 

 become progressively more conspicuous with the development of what 

 appear to be "rays," or "fibers," that radiate from them. The rays that 

 extend toward the equatorial plate of the nucleus, midway between the 

 poles, are particularly striking, and meet there as the common base of a 

 double cone with its apices in the centrioles. This figure is often spectacu- 

 lar in stained cells and has been given the name of amphiaster (double 

 star), each centriole with its rays suggesting a star. Actually the functions 

 of the centrioles and their apparent rays are not at all clear; cells that 

 lack centrioles and amphiaster accomplish the process of mitosis equally 

 as well as those which show them. 



The third main event of the prophase is the disappearance of the 

 nuclear membrane and the formation of a gelatinous spindle in much of 

 the area occupied by the nucleus. When asters are present they approxi- 

 mately mark the poles of the spindle, but the actual spindle appears to be 

 formed by a gelatinization of the colloidal nuclear sap, with a separate 

 spindle element, or segment, for each chromosome. By the time the spindle 

 is formed the centromeres of the chromosomes lie in the equatorial plate, 

 each within its own spindle element; but the ends of the chromosomes 

 often protrude well outside of the spindle into the surrounding cytoplasm. 

 The completion of these events marks the end of the prophase. 



Metaphase. The metaphase is comparatively brief. The single 

 centromeres that up to now have united the sister chromatids of each 

 chromosome divide, and the two halves of each centromere repel one 

 another. 



Anaphase. The divided halves of each centromere continue to repel 

 one another and move toward the opposite poles of the spindle. The 

 attached sister chromatids are thus gradually pulled apart, as the dis- 

 tance between their (half) centromeres increases. There is some evidence 

 that although the first stages of this separation are caused by the repul- 

 sion between the halved centromeres, the latter part of anaphase is to be 



