THE CELL AS A FUNDAMENTAL UNIT OF LIFE 



29 



which cells are duplicated in practically all forms of life. Since it is 

 so universal and so important a part of life it will be well to learn it 

 thoroughly before proceding on to a further study of animal life. 



Regulation of Mitosis 



What controls the rate of mitosis — why do cells sometimes become 

 duplicated rapidly when there is need for new cells and then cease 



Photos by Winchester 



Fig. 2.6. Photographic evidence of coiling of chromosomes in living plant and animal 

 tissue. Top left, prophase of Tradescantia (Spiderwort) showing coils. Top right, 

 later prophase of same plant showing the two complete sets of coils of the duplicated 

 chromosomes. Lower left, prophase from a cell of the grasshopper testes ; the double 

 coiling system can be seen in some places. Lower right, several chromosomes from 

 an anaphase of the same insect in which the chromosomes have been pulled out to 



show the coils. 



their duplication when this need is satisfied? In the answer to this 

 question lies the solution to many problems of biology. During the 

 early stages of the development of the human embryo the cells may 

 undergo mitosis every twelve hours or so. When the body is mature, 

 however, most of the cells cease duplication, as there is no longer 



