(2127). It will be seen that as yet the real aorta does not exist; for at this period of the metamorphosis all the blood passes through the vascular arches that remain into the dorsal vessel (fig. 374, m), which is formed in the same manner as the aorta of Fishes, by the union of the branchial vessels.

(2128). While the branchial fissures penetrated into the pharyngeal cavity, the branchial vessels were contained in the corresponding branchial arches; but as soon as these fissures disappear, the vascular trunks abandon the neighbourhood of the pharynx and begin to assume the character they afterwards present.

(2129). The most posterior arch of the left side gradually disappears, and on the seventh day of incubation is no longer recognizable; whilst in the meantime the current of blood from the right ventricle is directed in such a manner as to pass in front of this arch, and enters the posterior arch of the right side, and the last but one on the left.

(2130). As, moreover, the two arches that were formerly the most anterior have become obliterated, while the third and fourth, on the contrary, are increased in size, the blood, passing backwards through these arches into the roots of the aorta, enters also the carotid artery, which now resembles a prolongation of the commencement of the aorta towards the head. Thus, one part of the primitive root of the aorta becomes the trunk of the carotid artery.

(2131). There exist consequently, on the eighth day, three vascular arches on the right side, and only two on the left; and these five arches are derived from the heart, as are also two small vascular trunks, now quite distinct, which have been formed from the bulb.

(2132). The anterior arch of both sides and the middle arch of the right side proceed from the left ventricle; the posterior arches issue from the right; but all of them as yet unite to form the two roots of the aorta, which are still of pretty equal size, and each root gives off a carotid artery. At the point where the anterior arches join the roots of the aorta, they are now seen to give off newly-formed trunks, which go to the anterior extremity of their respective sides; and as these limbs and the head increase in size and require more blood, the anterior arch propels a greater proportion of blood in that direction, and insensibly less and less into the aorta. The consequence is that the anterior arch becomes more and more decidedly the brachio-cephalic trunk; and, in short, on the thirteenth day it only communicates with the dorsal aorta by a small vessel, and ultimately becomes quite detached, forming the arteria innominata of the corresponding side.

(2133). Meanwhile the posterior arches on both sides send out branches destined to the contiguous lungs. On the eighth day these vessels are still very small, and difficult to find; but they soon grow larger; and during the last half of the period of incubation, they show themselves as the immediate continuations of the arches from which they are derived, - their junctions with the aorta becoming more and more imperfect, and constituting the two ductus arteriosi. These canals are of very unequal size: that of the right side is much shorter than that of the left, which is now the only remnant of the original root of the aorta on that side, and considerably narrower than the root of the aorta on the right side. On the right side, in fact, the middle arch now becomes of great importance, and really constitutes the commencement of the descending aorta, receiving the other communications as subordinate parts.

(2134). The bird having escaped from the egg, and having breathed for some time, all the blood from the right ventricle passes into the lungs, the ductus arteriosi become totally imperforate, and two distinct circulations are thus established - one proceeding from the right side of the heart through the lungs into the left side of the heart, the other from the left side of the heart through the system into the right side of the heart.

Membranes of the ovum.

Fig. 375. Membranes of the ovum.

(2135). We see, therefore, that of the five pairs of vascular branchial arches which at first by their union formed the aorta as in Fishes, those of the first pair on both sides and of the fifth on the left side speedily disappear. The third on each side become the brachio-cephalic trunks, the fourth of the right side becomes the descending aorta, while the fifth of the right side and the fourth of the left side are converted into the pulmonary arteries. The very short trunk common to the two pulmonary arteries, as well as the equally short trunk of the aorta, properly so called, are produced by the transformation of the single cavity of the original "bulbus arteriosus" into two distinct canals, and thus this wonderful metamorphosis is accomplished.

(2136). About the one hundred-and-twentieth hour from the commencement of incubation, the vascular layer of the blastoderm has spread extensively over the yelk (fig. 375, b); and as the vessels formed by it become perfected, they are found to converge to the navel of the embryo, and to constitute a distinct system of arteries and veins (omphalo-mesenteric), communicating with the aorta and with the heart of the foetus, and forming a vascular circle surrounding the yelk. The omphalo-mesenteric arteries (fig. 375, b, c), which thus ramify over the vitelline sac, are derived from the mesenteric arteries; and the blood distributed through them is returned, by the omphalomesenteric veins, to the superior vena cava of the young chick.

Membranes of the ovum in situ.

Fig. 376. Membranes of the ovum in situ.

(2137). As soon as the intestinal system of the embryo bird is distinctly formed, the membrane enclosing the yelk (vitellicle) is seen to communicate with the intestine by a wide duct (ductus vitello-intestinalis), whereby the nutritive substance of the yelk enters the alimentary canal to serve as food, and the mucous membrane lining the vitellicle becomes thrown into close wavy folds, so as to present a very extensive surface. Gradually, as growth advances, the yelk diminishes in size; and at length, before the young bird is hatched, the remains of it are entirely withdrawn into the abdominal cavity (figs. 378,379), where its absorption is completed; but even in the adult bird, a little ca?cal appendage, or diverticulum, still indicates the place formerly occupied by the ductus vitello-intestinalis.