The _circulation of the blood_ is somewhat like the circulation in the fetus of quadrupeds. The primitive tubular heart (Fig. 83, No. _D_, 8) is bent in S-shape; the two ends are connected with blood-vessels; and later, by the development of the septa, the cavities of the adult heart are defined. The foramen ovale is found in the median septum. This opening brings the right and the left side into communication. The septa between the cavities of the heart are completed by the end of the sixth day. The two vitelline, or omphalo-mesenteric, veins carry the blood containing the nutrients from the yolk sac to the liver, where it is mixed with the blood drained from the intestines by the portal vein, and this blood is finally carried by the posterior vena cava into the right auricle of the heart. From here it passes through the foramen ovale to the left auricle; from the left auricle it enters the left ventricle, from which it is forced into the aorta, and then out into the systemic circulation. Practically all, if not all, of the blood of the pulmonary artery is sent into the aorta through a connection with this vessel called the ductus arteriosus. _Down_ appears about the thirteenth day of incubation. There are two kinds of down on the chick, one long, which comes first, about two or three days before hatching; a second, or fine, down forms at the roots of the other. As the embryo develops the air cell, at the large end of the egg which is developed between the two shell membranes, or _membranæ putaminæ_, gradually enlarges. OUTLINE FOR LABORATORY STUDY OF THE CHICK The objects of laboratory study of the embryos are as follows: 1. To study the living embryo. 2. To study the entire embryo: With the dissecting microscope, as an opaque object. With the compound microscope after killing, hardening, clarifying, and mounting. 3. To study embryos by dissection, in later stages. 4. To study the serial sections with the compound microscope. THE LIVING EMBRYO The egg is opened under warm physiological salt solution: 0.8 per cent. sodium chlorid in distilled water heated to a temperature of 38°C.