Most anatomical features of human embryos (Plate 2.1) are common to all vertebrate embryos, based on a pattern that emerged over 400 million years ago. Examples include the notochord, a collagenous rod that extends the length of the body axis, and somites, segments of mesoderm arranged on either side of the notochord. The tissues that will form these structures are produced by rearrangements of embryonic cells during a stage called gastrulation, when the embryo is less than two weeks old. These tissues are: (1) the ectoderm, which forms the skin and nervous system; (2) the mesoderm, which forms the musculoskeletal system and circulatory systems; and (3) the endoderm, which forms most of the digestive and respiratory systems. Very soon after gastrulation, organs begin to form. The first structures to appear are the primordia of the brain and spinal cord, backbone and muscles, digestive tube, and cardiovascular systems (Plate 2.2). These early organ systems will expand, a few additional ones added, and by the 10–12-mm stage, all organ systems are in place and most are functional. Which of these structures, based on their location and organization, resemble those found in the adult? For example, do you have somites and a notochord? If not, why are they present in all vertebrate embryos, including humans? The reason is phylogenetic constraint: We, humans, have an evolutionary history from our ancestors, and the organizational configuration that was laid down during early vertebrate evolution has remained with few modifications. The fact that embryos continue to adhere to this very ancient configuration is due to developmental constraint.