DEVELOPMENT. 45 



(4) When each plane is at right angle to the pre- 

 ceding plane and in two dimensions, a surface of cells 

 is formed, as simple epithelium. 



(5) When each plane is at right angle to the pre- 

 ceding plane and in three dimensions, a volume or 

 an organ is formed with three dimensions, as the 

 morula stage in embryos. 



General Considerations. The subject of cell divi- 

 sion has given rise to much discussion. While we 

 are unable to control mitosis, the following are 

 factors that modify cell growth : 



1. Trauma. Following a cut or a bruise the ad- 

 jacent cells are stimulated to rapid multiplication 

 and the wound heals. 



2. Heat. There is a mean temperature for the 

 maximum multiplication of cells which varies with 

 the species. In man this mean temperature is blood 

 heat, 98.6. 



3. Electricity. By stimulating protoplasmic ac- 

 tivity, electricity no doubt is a factor in influencing 

 normal cell growth. Just how this is accomplished, 

 and to what extent, is a much-disputed problem. 



4. Light. A very .important factor in promoting 

 multiplication, or decreasing it, or even destroying 

 certain cells, as bacteria. The different rays of 

 light have each a specific effect. 



5. Nourishment. Proper food is a stimulant. 



6. Chemical. Certain salt solutions, as sodium 

 and potassium salts, have a stimulating effect. 



7. Exercise. Regular systematic exercise prompts 

 a healthy growth. Massage acts in the same way. 

 A laborer, after a day of toil, is exhausted because 



