CELLS AND TISSUES 43 



from thirty to sixty minutes or more and the metaphase lasts only two 

 to six minutes. 



Anaphase. The chromosomes immediately separate (Fig. 3.5) and 

 one ot the separating daughter chromosomes goes to each pole. The 

 period during which the separating chromosomes move from the equa- 

 torial plate to the poles is known as the anaphase and lasts some three 

 to fifteen minutes. The spindle fibers apparently act as guide rails along 

 which the chromosomes move toward the poles. Without such guide rails 

 the chromosomes would merely be pushed randomly apart and many 

 would fail to be incorporated into the proper daughter nucleus. The 

 mechanism by which the chromosomes are moved apart is not clear. 

 Experiments suggest that the protoplasm between the chromosomes takes 

 up water, swells, and pushes the chromosomes apart. Other experiments 

 indicate that some of the spindle fibers are contractile and can pull the 

 chromosomes toward the poles. 



Telophase. When the chromosomes have reached the poles of the 

 cell, the last phase of mitosis, the telophase, begins. Several processes 

 occur simultaneously in this period: a nuclear membrane forms around 

 the group of chromosomes at each pole, the chromosomes elongate, stain 

 less darkly, and return to the resting condition in which only irregular 

 chromatin threads are visible, and the cytoplasm of the cell begins to 

 divide. Division of the cytoplasm is accomplished in animal cells by the 

 formation of a furrow which circles the cell at the equatorial plate and 

 gradually deepens until the two halves of the cell are separated as inde- 

 pendent daughter cells. The events of telophase require some thirty to 

 sixty minutes for their completion. 



The mitotic process results in the formation of two daughter cells 

 from a single parent cell with each daughter cell having exactly the same 

 number and kind of chromosomes, and of the units of heredity (genes) 

 contained in these chromosomes, as the parent cell. Since all the cells 

 of the body are formed by mitosis from a single fertilized egg, each cell 

 has the same number and kind of chromosomes, and the same number 

 and kind of genes, as every other cell. 



The speed and frequency of cell division vary greatly from tissue to 

 tissue and from one animal to another. In the early stages of embryonic 

 development, there may be only thirty minutes or so between successive 

 cell divisions. In certain advdt tissues, notably the nervous system, mitoses 

 are extremely rare. In other adult tissues, such as the red bone marrow, 

 where red blood cells are produced, mitotic divisions must occur fre- 

 quently to supply the 10,000,000 red blood cells each human being 

 produces every second of the day and night. 



Regulation of Mitosis. The factors which initiate and control cell 

 division are not certain. Tlie possible role of the ratio of cell surface to 

 cell volume was discussed previously (p. 39). The ratio of nuclear sur- 

 face to nuclear volume may also be important. Since normal cell function 

 requires the transport of substances back and forth through the nuclear 

 membrane, growth will eventually result in a state in which the area of 

 the nuclear membrane is insufficient to meet the demands of the volume 

 of cytoplasm. Cell division, by splitting the volume of cytoplasm into 



