Cells and Their Activities 271 



tudinally, increase in thickness, and shorten gradually to form the 

 chromosomes. As the chromosomes appear they first consist of two fila- 

 ments formed from the chromonemata. The gradual condensation and 

 shortening plus extra material results in the formation of the chromo- 

 somes. Each chromosome consists of two chromatids closely associated 

 throughout their length. The centrioles at this same time gradually 

 separate, one migrating toward either end of the cell. The centriole 

 is accompanied by a system of radiations called asters or astral rays, 

 and between the two centrioles a bundle of filaments, the spindle, gradu- 

 ally arises. 



Late Prophase. — By the end of early prophase, the chromosomes 

 are well formed and the spindle is beginning to be in position. This is 

 followed by the disintegration of the nuclear membrane, the complete 

 formation of the spindle, the disappearance of the nucleolus, and the 

 convergence of the chromosomes toward the center of the cell. It is 

 fairly evident during this late prophase that each chromosome is a 

 double structure. 



Metaphase. — This phase occupies but a short period of time and 

 marks a point of equilibrium between the convergence of the chromo- 

 somes and their movement toward the centrioles or poles. The double 

 structure of the chromosomes is clearly evident at this time. The posi- 

 tion of the chromosomes upon the spindle is here called the equatorial 

 plate. The chromosomes are actually at the edge of the cell arranged 

 in a radial manner upon the equator of the spindle. This is clearly evi- 

 dent from a polar view of the pattern. 



Anaphase. — The equilibrium characteristic of the metaphase 

 rapidly disappears as the chromosomes start moving away from the 

 equatorial plate toward the asters. This movement seems to be facili- 

 tated by the gradual shortening of the spindle fibers. As each chromo- 

 some is attached to a spindle fiber, this results in their moving pole- 

 ward. The movement is also assisted by an apparent repulsion between 

 the two chromosomes. These chromosomes are termed daughter 

 chromosomes when they start separating. Each daughter chromosome 

 is one of the former chromatids. 



Telophase. — The end of this polar migration marks the start of 

 the telophase. The chromosomes gradually lose their identity and the 

 nucleus starts reforming. This is followed by the constriction of the 

 cytoplasm in the equatorial region. As this constriction deepens, the 



