.c.), and two posterior cardinal veins
(p.c.) uniting to form Cuvierian sinuses (c.s.) that open into the heart
just as in the dog-fish. But later the inferior cava is developed and
extends backward, the posterior cardinals atrophy, the Cuvierian
sinuses become the superior cavae, and the anterior cardinals the
internal jugular veins. The vitelline veins (v.v.) flow, at first,
uninterruptedly through the liver to the inferior cava, but, as
development proceeds, a capillary system is established in the liver,
and the through communication, the ductus venosus, is reduced-- at
last-- completely. Bearing in mind that the yolk is outside the body in
the fowl and inside it in the frog, the vitelline veins of the former
have a considerable resemblance in position, and in their relation to the
portal vein, to a portion of the single anterior abdominal vein. Blood is
taken out to the allantois, however, by the arteries of the latter type.

Section 32. Five aortic arches are generally stated to appear
altogether in the fowl, but not simultaneously. The first two, the
mandibular and the hyoid vascular arches, early disappear, and are
not comparable to any in the frog. The third is the first branchial arch,
and, like the corresponding arch in the frog, forms the carotid artery;
the second branchial is the aortic arch; and what has hitherto been
regarded as the third (the fifth arch, i.e.) the pulmonary artery. A
transitory arch, it is now known, however, appears between the
second branchial and the last, and it is therefore the fourth branchial
arch which is the pulmonary, just as it is in the frog.

Section 33. Blood, it may be mentioned, first appears in the area
vasculosa, the outer portion of the area opaca. Embryonic cells send
out processes, and so become multipolar; the processes of adjacent
cells coalesce. The nucleus divides, and empty spaces appear in the
substance of each of the cells.

In this way, the cavities of the smaller vessels and capillaries are
formed, and the products of the internal divisions of the cells become
the corpuscles within the vessels. The red blood corpuscles of the
rabbit, it may be added, are nucleated for a considerable portion of
embryonic life. Larger vessels and the heart are burrowed, as it were,
out of masses of mesoblast cells. The course of the blood in the
embryo is by the veins to the right auricle, thence through the
imperfection of the auricular septum already alluded to, into