This chapter describes the embryonic period and explains its importance in human development. It discusses cleavage and its contribution to development. The period of cleavage is characterized by the series of mitotic cell divisions by which the organism acquires the property of multicellularity. The chapter explores the significance of gastrulation in development and shows the origin of each of the following: primitive streak, primitive node, notochord, endoderm, mesoderm, ectoderm. Human eggs undergo holoblastic cleavage, resulting in cells that are of equal size. The chapter discusses the significance of the notochord in animal classification and development, including the following terms: chordata, chordate, vertebral column, vertebrates, embryonic induction. It also describes the process of implantation, including the following terms: implantation, nidation, syncytiotrophoblast, cytotrophoblast, lacunae, placenta, and intervillous spaces. The amniotic cavity has a roof and walls of cytotrophoblast and a floor of the upper layer of the embryonic disc.