In the fourth week all these parts advance. The atrium cavity of the heart begins to divide; the alimentary tract shows the pharynx and esophagus, stomach, and gut; the pancreas starts, the liver diverticulum divides, and the bile-ducts appear. The lung _anlage_ bifurcates and the primitive trachea is seen. The ventral roots of the spinal nerves appear, the interior ear is indicated, and the eye is deeper. The buds of the legs and arms appear about the twenty-first day--by the thirty-second day even the fingers are present. The four heart-cavities are formed, the intestinal canal is nearly closed, the first indications of the liver and kidneys appear. The child now has reached the fetal stage, and its living body is made up of myriads of cells all derived from the original fertilized ovum. The fetus is then one centimetre, or two-fifths of an inch, in length--about the length of the word "fetus" here. At the end of the second month the fetus is two and a half centimetres long. The ears appear, and the tail-like process at the lower end of the spine disappears. The arms show the three parts, arm, forearm, and hand; and a little later the thigh, leg, and foot are differentiated. The navel begins to close, the liver develops, the abdomen is yet partly open. At the end of the third lunar month the fetus is seven to nine centimetres long. The intestinal canal is formed and contains bile. The body resembles that of a human being, but the head is proportionately very large. Bony tissue begins to appear. [Illustration: FIG. III. The Development of the Fetus.] At the end of the fourth lunar month the fetus is ten to seventeen centimetres long. Some muscles are movable. The heart-beat is strong. Sex is distinguishable externally. The skin is bright red, and so transparent that the blood-vessels are visible through it. Toward the close of the fifth lunar month the head is about the size of a hen's egg. The skin is somewhat less transparent. There are indications of hair and nails. The eyelids are closed. Parts of the brain and spinal cord are formed. Such a fetus may live for five or ten minutes if removed from the womb, and it may make attempts at respiration. At the end of the sixth lunar month the fetus, if born, may live for several hours under favorable circumstances. Its respiratory, digestive, and related organs are not developed, and no artificial feeding will keep such a child alive. The brain cortex, the organ of consciousness, begins to laminate into three strata of nerve-cells at the beginning of the sixth month.