Once the two types of gametes have completed the necessary maturation steps, they are capable of uniting to form a zygote and initiating embryogenesis. As may be suspected from the complexity just observed in the steps of generating the eggs and sperm, neither the union of those gametes nor the formation of the embryo is a simple event, with little variation from taxon to taxon; rather, both are as complex and varied as the major groups of organisms themselves. Yet a number of features common to all are found to pervade most of the sexually reproducing biotic world. Because of their abundance, size, ease of culture, synchronous division, and other favorable attributes, echinoderm eggs have provided the basis for the great bulk of the investigations into these phases of development, but studies on vertebrates and metaphytans have served to enrich the literature to a considerable extent. In the present chapter, the penetration of the sperm into the ovum and related events are followed, while the development of the early embryonic stages and then their subsequent differentiation are the respective provinces of the following two chapters.