This chapter explains how the sagittal, frontal, and transverse planes divide the human body. A large part of the content of human developmental biology is developmental anatomy. The anatomic position for a human is standing straight and upright, with arms at the sides and palms facing forward. An important feature of an animal's anatomy is its symmetry. The chapter explores the difference between radial symmetry and bilateral symmetry. In addition to anatomic descriptions used to describe the developing human form, some terminology is necessary to describe the progression from fertilization to birth. The chapter also explains the differences between the developmental timelines used by obstetricians and embryologists, and describes comparative embryology. Comparative embryology looks at the way in which the human egg divides, explaining why humans begin their development differently from the way other animals do.