Introduction

Every organism, however large, begins life as a single cell. That one cell is somehow capable of giving rise to an enormous, complex body made of millions of cells — so how does this happen? The answer lies in two closely linked abilities that all living cells share: growth and reproduction. A cell reproduces by dividing into two, so that one parental cell gives rise to two daughter cells each time it divides. Each of these newly formed daughter cells can then grow in its own right and divide again, producing further generations of cells and steadily building up a growing cell population. Repeated over and over, this cycle of growth followed by division is exactly what allows a single starting cell to eventually form a structure made up of millions of cells — the basis of how every large, multicellular organism develops from one fertilised cell.