Embryonic development converts a single-celled zygote into a complex, multicellular organism with specialized tissues and organs. Whether this development takes place rapidly or slowly, and whether in a naked egg, an amniotic egg, or within the mother's body, the process involves a sequence of similar steps in all vertebrates. We began our examination of vertebrate biology by looking first at the diversity of vertebrates that have evolved on the earth, and then at the structure and functioning of their various tissues and organ systems. This chapter explains how each vertebrate winds up with particular tissues, organs, and body parts arranged the way they are. Sexual reproduction produces a new vertebrate; development molds it into its proper form and function.