When the heart has assumed its form as such, distinct from the great trunks rising from it, the two _arteries_ from the base of the ventricles appear. The artery to the right bifurcates, one division supplying the head and the wings, the other winding over the right bronchus. That to the left also bifurcates. Its left division arching over the left bronchus and anastomosing with the right arch a little below and behind the apex of the heart. Its right division arches over the back of the heart, bending to the right and anastomosing with the right aortic arch just above the outer ductus arteriosus. Each of these divisions of the left primary arterial trunks sends off a branch to its corresponding lung. As the lung expands, and especially at the beginning of the act of expansion toward the close of the period of incubation, the blood is diverted into the pulmonary vessels, and the channels below them shrink and disappear. The left primary artery is retained as the trunk of the pulmonaries, and, through the changes in the interior of the ventricle, this artery comes to discharge exclusively the ventricle corresponding to the right in mammals. The retained aorta rises from the left ventricle. The _air-sacs_ begin at the lower point of the lung, appearing like small hydatids, and extend further and further into the abdomen, in front of the kidneys. They are at first full of fluid. Soon after the development of the abdominal air-sacs others are developed. The _lungs_ are at first free, but afterward begin to be attached to the ribs and to the spine. In the female embryo we first observe two _oviducts_, one on each side of the basis, or stroma of the ovarium, which appears in a relation to the primordial kidneys similar to that of the testes in the male. At the period when the permanent kidneys have sent their ureters to the cloaca, the oviducts have been developed as prolongations from that part, and, up to a certain point of development, they are of equal size and length. Subsequently the left oviduct alone continues to grow; the right remaining stationary or shrivels; occasionally it may be discerned as a rudimentary in the mature bird, but usually all trace of it has disappeared before hatching. The left oviduct expands above or at its free end into the infundibular orifice, where its parietes are very thin. As it descends, these increase in thickness, and the efferent tube gradually acquires the texture and form of an intestine. It is attached and supported by a duplicature of the peritoneum. [Illustration: FIG. 82.—Embryological studies. _A._ 1, The chorion and allantois. 2, The allantoic cavity. 3, The amnion. 4, The yolk sac. 5, A small quantity of remaining albumin. _B._ Brain tube of a chick 25½ hours old showing partly closed brain tube with eleven folds of neuromeres.