MITOSIS 



689 



the metaphase chromosome. At the beginning of the third stage, 

 or anaphase, the part of the chromosome attached to the spindle, 

 called the centromere, divides, and its halves move towards 



Fig. 536. — Schematic history of two homologous chromosomes during mitosis. 

 From Callan, New Biology, No. 7, 1949. Penguin Books. 



a. Resting nucleus surrounded by a nuclear membrane and containing a nucleolus and two aggregations 



of nucleic acid. 

 6, The chromosomes as they first appear inside the nucleus ; each consists of two chromatids, and the 



centromeres are represented by gaps. 



c, The chromosomes have contracted in length and increased in thickness (approaching end of prophase). 



d, The nuclear membrane has disappeared, the spindle has formed, and the chromosomes lie with their 



centromeres on the equator (metaphase). 



e, The centromeres have divided and are moving towards opposite poles of the spindle, carrying single 



chromatids with them (anaphase). 

 /, The spindle has elongated and the movement of the chromatids continues. The cytoplasmic cleavage 



furrow which will divide the cell into two is approaching from the sides, 

 g, The cleavage furrow is completed, and the daughter nuclei are being reconstituted ; the chromatids 



have now become chromosomes (telophase). 



the poles. As each goes it takes with it the chromatid of which 

 it is part ; in this way the chromatids are separated and become 

 chromosomes. In the fourth and last stage, or telophase, the coils 

 of the new chromosomes begin to unwind, and the nuclear 

 membrane is reformed. The chromosomes finally become invisible 



