30 THE CELL AS A FUNDAMENTAL UNIT OF LIFE 



need for more tissue. Some cells, such as those of the brain and nerves, 

 never undergo mitosis again. Other cells, such as those of the bones, 

 do not reproduce unless there is an injury, in which case they will pro- 

 duce new cells until the injury is repaired and then they cease re- 

 production. Still others, such as those of the skin and the blood-pro- 

 ducing cells, continue mitosis throughout life and thus replace the 

 skin cells and the blood cells which are destroyed in the course of our 

 daily living. If an injury should destroy some of the skin, the rate 

 of mitosis in the surrounding skin cells will become increased until 

 the damage is repaired ; then the rate will be reduced to normal. The 

 same is true of the blood-producing cells when there has been extensive 

 loss of blood through bleeding. In other words, there seems to be 

 some force which stimulates cells to undergo mitosis when new ones are 

 needed and prevents mitosis when there is no need for new cells. 



An explanation of the method of regulation of mitosis is of vital 

 interest, for here may lie the clue to the solution of one of the greatest 

 medical problems of our times — cancer. This dread affliction, which so 

 often turns a person's later years into a nightmare of pain and suffering, 

 is becoming more prevalent as the human life span is increased. In 

 cancer the factor which causes mitosis to cease when there is no need 

 for new cells is somehow lacking and there is a rapid, uncontrolled 

 rate of cell reproduction. This forms a mass of tissue which is outlaw 

 tissue — cells growing wild — cells out of control — cells which are para- 

 sites on the body of which they were once a normal part. Up to the 

 present time the most effective way to stop these mitoses and kill the 

 cells has been through the use of x-rays and radium. However, much of 

 the cancer research today is directed at an effort to find the factor 

 which regulates mitosis. The increased rate of cell duplication which 

 follows injury seems to be due to wound hormones produced by the 

 injured and dead cells. This has been demonstrated in plant tissues 

 and is thought to be true of animal tissues also. Also certain chemicals 

 have been discovered which stimulate or inhibit mitosis in experi- 

 mental tissue. If some method is found to apply mitosis-inhibiting 

 chemicals to cancer tissue or to apply chemicals which will neutralize 

 the mitosis-stimulating substance, we might be near a solution to the 

 cancer problem. 



A Problem of Chromosome Number 



The number of chromosomes found in the cells of plants and ani- 

 mals is constant among all members of the same species. There will 

 be variations in the genes in the chromosomes which account for the 



