Summary: , and senses. These functions are intentsely characteristic behaviors of the animal, but they are only manifestations of a much deeper underlying process that goes on throughout the whole life of the animal, the process of growth and differentiation. This process is best studied in the earliest stages of the animal's life; these are the stages that embryologists deal with most intimately. Their results are, however, of broader significance; for even to understand the behavior of the adult animal, one must understand something of the processes of its early development. This is why embryology lies at the very heart of the study of animal behavior and animal existence.