5. With the third week the stage of the embryo, technically so called, begins. During this week the body of the embryo is indicated. There are three layers of cells, already mentioned, the ectoderm, mesoderm, and endoderm, and these lie on the floor of the enveloping Amnion. The amnion is a loose fluid-filled sac (the caul) enveloping the fetus to protect it from jarring. The fluid in it is the "waters" that escape in parturition when the infant breaks through the caul. The archenteron in the third week shows the beginning of a division into two parts: the part that will go to the body proper of the embryo, and the part outside the body of the embryo which will form the yolk-sac, or umbilical vesicle, from which the embryo will draw sustenance until the placental vessels have been formed. The part of the archenteron that remains within the embryo proper begins in this third week to be moulded into the head-cavity. The forepart of the archenteron will later make the alimentary tract from the mouth to the middle of the duodenum, or small intestine beyond the stomach. The other part of the archenteron wall make the Allantois, the hind gut and the bladder. The allantois becomes a part of the fetal umbilical cord after the formation of the placenta. During this third week the dorsal outline of the embryo is concave; the heart has a single cavity, which will begin to divide during the fourth week; the vitelline blood circulation begins, and the blood-vessels of the visceral arch are laid down. The digestive system is advanced to a gut-tract, which is a straight tube connected with the yolk-sac. The liver evagination is present and the oral pit is a five-sided fossa. The respiratory system is represented by the _anlage_ of the lungs, a longitudinal protrusion of the ventral wall of the esophagus. The genito-urinary system begins as the Wolffian bodies. The mesoderm starts to segment to form the skin, and the neural canal (from which develop the spinal cord and brain) for the nervous system forms. The fourth ventricle of the brain is indicated, and the vesicles of the fore brain, mid brain, and hind brain are recognizable. The ears, nose, and eyes, muscular system, skeleton, and limbs are also beginning to be recognizable. At about the sixteenth or eighteenth day of gestation the various parts of the embryo rapidly differentiate.