During gestation-the period of nine months between fertilization and the birth of the infant-a remarkable series of developmental changes occur. Through the process of mitosis, the total number of cells changes from one (the fertilized egg) to about 2 ´ 1011. In addition, these cells differentiate into hundreds of different types with specific functions (liver cells, nerve cells, muscle cells, etc.). A multitude of regulatory processes, both genetically and environmentally controlled, accomplish this differentiation.

Francis Collins