The mode o f action o f genes during development can be investigated in two ways: (i) by experiments on the mechanism o f development, (2) by examination o f the changes produced in developing organisms by gene-changes. The first field o f inquiry is usually considered to constitute the separate science o f experimental embryology, while the second is a part o f genetics, and is often referred to as phaenogenetics. Such a separation is, however, very artificial; both subjects fall into the general field o f investigation o f how an adult organism arises from the individuals o f the previous generation. Thus some at least o f the results o f experimental embryology are essential for a full understanding o f genetics, and in this chapter we shall consider the general mechanism o f the development o f animals and the ways in which genes may act to control the course o f the reactions involved. Unfortunately there are no adequate data for a similar survey o f genetic control o f plant development.