The vessels which return the blood from the vitellicle are the transverse and the longitudinal _vitelline veins_. The first are so called because these trunks pass to the embryo at right angles to its axis. They are the largest returning canals. The _longitudinal veins_ extend parallel with the axis of the embryo; they are of smaller size. The right anterior longitudinal vein becomes the right precaval and receives the remains of the right transverse vitelline vein, as the right vena azygos. The left anterior longitudinal vitelline vein is also persistent as the left precaval, and enters in the mature bird, as in the embryo, at the posterior or the lower part of the auricle. The left transverse vitelline vein is also subsequently reduced, by receiving only the vertebral veins of that side, to the condition of a so-called azygos vein. The main trunk of the post-caval is the result of the returning vessels from the abdominal viscera and the posterior limbs at a later stage of development. There is but one principal posterior longitudinal vitelline vein, and this anastomoses with the left transverse vein as it enters the embryo. The _auricle_, which, by its dilatation of the left side, appears to be double, receives the venous blood at its right division. The left one, subsequently receives the veins from the lungs, is ultimately separated from the left precaval and the right auricle to which that vein is conducted and restricted. The _ventricular part of the heart_, at the second day of incubation, is in the form of a bent tube, curving from behind downward, forward, to the right and upward, continuing insensibly into the part representing the aortic bulb, in which the septum first appears, and ultimately dividing the ventricle into two. At this stage the piers of the _maxillary arch_ appear as buds from beneath the eyeballs. The naso-premaxillary process is above their interspace. The piers of the mandibular arch and those of the hyoidean arch follow in close succession. The blastemal base of the scapular arch projects slightly at the sides of the fovea cardiaca; the piers, now separate, ultimately meet in front of the heart, and accompany it in its retrograde course. The mesencephalon is the largest of the segments of the brain which are connected with the eyeballs.