Development of the body form

Up until and including S8, the embryo looks like a disc. The amnion with the amniotic cavity emerges over its dorsal surface. The embryo eventually covers the umbilical vesicle ventrally. At S5, the embryonic disk is still circular in shape. At S6, it begins to expand at its free cranial end and tapers off at its caudal end, where it is connected to the placenta through the connecting stalk. From S7, it lengthens to take on a pear-shaped appearance. It also grows thicker with the formation of the mesoderm (primitive streak, primitive node) and the development of the neural furrow, neural folds and neural tube. From S9, the still-flat embryonic disc is transformed into the embryonic body. The most important processes in this transformation are the cranial and caudal flexions, and the increases in volume of the inner structures. The embryonic body finally becomes completely covered with ectoderm, which extends into the amnion at the level of the umbilicus.