Fourth, the first liver diverticulum, and, at a short distance posterior to this, the second liver diverticulum. Fifth, the pancreas is at a point where the intestine opens ventrally. It first appears as a slight thickening of the dorsal angle of the intestine. Sixth, the ventral wall of the hind-gut forms a wide evagination. Seventh, the beginning of the allantois. After the whole series of transverse sections have been studied and drawn, construct a longitudinal section of the fetus, including a reconstruction of the alimentary tract. At this stage the =heart= is a simple tube. The following divisions are distinctly visible: the auricular portion, the ventricular portion, the sinus venosus, and the bulbus arteriosus. The union of the two ductus Cuvieri and the ductus venosus form the sinus venosus. The two ductus Cuvieri are formed by the union of the anterior and the posterior cardinal veins. The ductus venosus is formed by the union of the small right and the large left vitelline vein. These latter veins return the blood from the yolk sac. The sinus venosus empties into the single auricle above which it is located. The single auricle is later divided into two chambers, the right and the left auricle. At this stage it is widest in the lateral direction. The auricle empties directly into the ventricle. The ventricle lies ventrally and behind the auricle. This location is due to the bending of the heart at this stage of development. Its hindmost portion forms the future apex of the heart. If the series of sections be studied from the posterior forward, the ventricle will be first to appear in the sections. Just beneath the auricular portion of the heart there is the bulbus arteriosus. The bulbus arteriosus soon divides into a number of aortic arches. There is an ascending pair in each of the visceral arches. The dorsal aorta is formed by the union of the aortic arches above the visceral arches. The aortic arches are first continued a short distance forward as the carotid arteries. The dorsal, or posterior, aorta passes backward under the notochord. The dorsal aorta divides into two parallel aortæ which give off on each side the vitelline arteries. Note other branches of this aorta. The veins at this stage consist of the anterior and posterior cardinal, the ductus venosus, the ductus Cuvieri, and the vitelline veins. Make drawing of the circulatory system after a completion of the study of the series of sections. Make a study of the =nervous system= according to hints already given.