cular area known as the sinus terminalis. The sinus terminalis opens partly into the right and partly into the left omphalomesenteric veins, which subsequently unite into a common venous trunk, called the sinus venosus, which is continuous with the primitive auricle.

This vitelline circulation in the human embryo persists but a short time. After the fifth or sixth week of fœtal life it becomes obliterated, the yolk then being atrophied, and the placental circulation well developed.

(b) The later or placental circulation is developed in the mesoblastic layer of the allantois, especially in that part which is in relation with the decidua serotina. The allantois, when fully developed, extends to the chorion, over which it spreads, sending in processes to occupy the villi. These chorionic villi are embedded in the decidua of the uterus, and are especially developed at the upper part, which is in connection with the decidua serotina or maternal placenta.

The primitive aortæ, which were at first two separate tubes, become united in the dorsal region of the embryo, so that the two aortic arches end in a single vessel, which extends to the middle of the embryo, and there divides into two branches, each of which gives off a vessel called the vitelline or omphalomesenteric artery.

From the branches of the aortæ arise two large vessels, which, running along the allantois, spread out over the chorion, being especially directed to the upper part of this membrane; these are the umbilical or hypogastric arteries, which carry the blood from the aorta to the foetal placenta.

Veins arise from the terminal networks of these arteries, and combine to form the two umbilical veins. The umbilical veins take a similar course to the arteries, and convey the blood to the venous trunk formed by the junction of the omphalo-mesenteric veins.

After a time the right umbilical and right omphalo-mesenteric veins disappear, while from the trunk formed by the junction of the left umbilical and left omphalo-mesenteric veins, branches are given off to the liver (vene advehentes), and at a point

nearer the heart, vessels are received from the liver (venæ revehentes).

To the part of the vessel intervening between the origin of the venæ advehentes and the entrance of the venæ revehentes is given the name of the ductus venosus.

Thus it may be seen that in the placental circulation the blood is conveyed from the aorta, by the umbilical arteries, to the foetal placenta, undergoes changes, owing to its close relation

Diagram of the heart and principal arteries of the chick. (Allen Thomson.) B. and C. are later than A.

1, 1. Omphalo-mesenteric veins. 2. Auricle. 3. Ventricle.

tive aorta. 6, 6. Omphalo-mesenteric arteries.

4. Aortic bulb.
5, 5. Primi-
A. United Aorta.

ship to the maternal blood. From the placenta it is returned by the umbilical vein, which sends a part through the liver and a part direct to the heart. The more minute details of fœtal circulation will be described later on.

The Arterial System.-Around the pharynx are developed five pairs of aortic arches. These commence anteriorly from the two primitive aortæ, and, passing along the side of the pharynx, end in the aortæ as they descend to become united in the dorsal

region of the embryo. The points of origin of the arches are termed their anterior roots, and the points of termination their posterior roots.

I, II, III, IV. Pharyngeal clefts of the left side, showing the relationship of the clefts to the aortic arches.

A. Aorta. P. Pulmonary artery. d. Ductus arteriosus. a'. Left aortic root. a. Right aortic root. A'. Descending aorta. pn. pn'. Right and left vagi. s. s'. Right and left subclavian arteries. v. v. Right and left vertebral arteries. c. Common carotid arteries. ce. External carotid. ci ci. Right and left internal carotid.

Though all these arches do not exist at the same time, still, in describing the vessels which arise from them, they may be conveniently considered together.

On the right side the fifth arch disappears completely. On the left side the anterior root and neighboring part of the fifth arch are transformed into the pulmonary artery; the remaining part of this arch continues as the ductus arteriosus, which connects the pulmonary artery with the permanent aorta.

The fourth left arch, in mammalia, becomes the permanent aorta. At the junc

tion of the fourth and fifth left posterior roots the left subclavian artery is given off. In birds the right fourth arch is transformed into the perma

nent aorta; and in examining the development of the aortic

arch of the chick, it must be borne in mind that it is on the opposite side to that it occupies in man.

On the right side the anterior root of the fourth arch, and the part of the aortic trunk leading to it, persist as the innominate artery, the fourth arch being represented by the right subclavian artery.

The part of the primitive aortic trunk joining the fourth and

FIG. 308.

az

ca

third right anterior roots becomes the common carotid artery of the same side, while arising from this is the internal carotid, which, taking the position

of the third arch, passes to the posterior roots, and occupies the trunk of the primitive aorta from the third to the first arches. . The external carotid, arising from the common carotid at the third anterior root, occupies the position of the vessel joining this root to those of the second and first arch.

On the left side the common carotid and its branches are developed similarly to those on the right, the only difference being that the common

ca

ac

B

carotid arises from the

ELLL

u

aorta and not from the

innominate.

u

m

The iliac arteries are 4. Plan of principal veins of the fœtus of about four developed from the hypogastric. At first they appear as branches, but with the growth of the limbs they become so much larger that after birth. Mesenteric veins. az. Azygos vein. ca'. Remains they appear to be main branches from the point of division of the

weeks old. B. Veins of the liver at an earlier

period. C. Veins after the establishment of the

placental circulation. D. Veins of the liver at the same period.

Primitive jugular veins. dc. Ducts of Cuvier.

ca. Cardinal veins. ci. Inferior vena cava. . Duc

tus venosus. u. Umbilical vein. p. Portal vein.

o. Vitelline vein. cr. External iliac veins. o. Right vitelline vein. u'. Right umbilical vein. . Hepatic veins (venæ revehentes). pp. Venæ advehentes.

of left cardinal vein. li. Cross branch from left

the jugular, which becomes the left brachio-cephalic vein. ri. Right innominate vein. s.s. Subclavian veins. h. Hypogastric veins. i. Division of inferior vena cava into the common iliac veins.

aorta, the hypogastric arteries now being merely small branches of the iliac vessels.

With the development of the organs and limbs, vessels in connection with those above described arise in the mesoblast. It is, however, beyond the scope of this work to describe in detail the origin of the lesser vessels.

Venous System.-The blood is returned from the head by the two primitive jugulars, which unite with the cardinal veins conveying the blood from the trunk and lower extremities to form a vessel on each side, called the duct of Cuvier.

From the lower extremity of the embryo the inferior vena cava commences by the union of the external iliac veins; this passes up and opens into the venous trunk common to the left vitelline and left umbilical veins.

The left vitelline becomes continuous with the vessels from the common trunk going to the right side of the liver (the right vena advehens), and forms the main trunk of the portal vein (Fig. 308, B. and D.).

At this stage of the formation of the veins there are three trunks opening into the auricle, the right and left ducts of Cuvier and the inferior vena cava.

As development proceeds, the lower parts of the cardinal veins join the external iliac veins, forming the common iliacs, and so return their blood into the inferior vena cava.

The upper parts of the cardinal veins become continuous with the posterior vertebral veins which convey the blood from the parietes of the embryo. Between the latter a communicating branch is established, which helps in the formation of the azygos vein.

The ducts of Cuvier, which at first were placed almost at right angles to the auricle, become more oblique in their direction as the heart descends.

Between the primitive jugular veins a cross branch is developed, which conveys the blood from the left side of the head and upper extremity of the duct of Cuvier of the opposite side.

The left duct of Cuvier, below the communicating branch, atrophies and forms part of the coronary veins of the heart.