THE CELL 19 



7-'ro;;//fl.s'r. This phase of nuclear actixity is preceded and 

 accompanied by activity of the two centrioles in the center of the 

 cytocentruni or attraction s])here. When activity })egins, the 

 centrioles ni()\'e a])art until they occui)y a position op})osite each 

 other 90 degrees from their resting position, with the nucleus 

 between them. Astral rays a])pear in the cytoplasm, radiating out 

 from each centriole. In pre])arations, these rays often resemble 

 fibers. Some of them form a fibrillar apparatus between the two 

 centrioles, which is known as the achromatic s])indle, or, together 

 with the centrioles, the amphiaster. Simultaneously the nuclear 

 membrane and nucleolus disappear, and nucleoplasm mingles with 

 cytoplasm. The distributed chromatin granules of the resting 

 nucleus are api)arently organized into a long, coiled, tangled thread, 

 which shortens through condensation of its constituents and finally 

 forms a specific numlier of pieces, or chromosomes. The num})er of 

 chromosomes forming at this stage is usually constant for all somatic 

 cells of each species. The chromosomes move so that they lie on 

 the sijindle in an imaginary })lane midway between the two cen- 

 trioles at right angles to the axis. Longitudinal splits in the chromo- 

 somes now become api)arent as grooves, though they may actually 

 have split much earlier in the prophase stage, and some of the astral 

 rays (fi})ers) ai)i)ear to attach to chromosomes. 



Metaphase.^Dwv'm^ this second phase longitudinal splitting is 

 completed and each chromosome is found to be equally divided 

 into two longitudinal halves. The two halves of each chromosome 

 separate as they move toward their respective centrioles, a feature 

 marking the beginning of the anaphase. 



Anaphase. — In this third stage, the chromosomes continue toward 

 the centrioles. In late anaphase a groove api)ears in the cell mem- 

 brane corresponding with the location of the equatorial ])lane, which 

 is marked by a granular line. This groove deepens as the chromo- 

 somes pass to opposite poles. The chromosomes assemble at 

 opposite poles and initiate the telophase stage. 



Telophase. — hi the final phase the equatorial constriction, or 

 groove, continues so that eventually 2 daughter cells are formed, 

 each with its own cell membrane. Two daughter nuclei are organ- 

 ized in the same way as was the original resting nucleus, though 

 they are smaller in size. In this process, a nuclear membrane forms 

 about each daughter nucleus, and the chromatin becomes flistributed 

 in it as it^was in the resting nucleus of the parent cell. A nucleolus 



