The study of life is the study of tier upon tier of interlocking homeostatic mechanisms, and the main theme of this book concerns that part of the system that ensures cellular and tissue homeostasis and thus maintains tissue mass, tissue structure and tissue function in the adult mammal. The functional existence of any adult tissue depends on the continuing control of the states of differentiation of its cells. Just as in an embryo, where cellular differentiation is initiated and controlled by the genetic responses of the cells to specific messenger molecules, so also throughout the whole of...