The connection between this and the vein above the cross branch being, in the adult, represented by a small vein, or a band of fibrous tissue, called the vestigial fold of the pericardium.

The cross branch from the left to the right jugular becomes

FIG. 309.

Diagram illustrating the circulation through the heart and the principal vessels of a

a. Umbilical vein.

foetus. (Cleland.)

b. Ductus venosus. f. Portal vein. e. Vessels to the viscera. d. Hypogastric arteries. c. Ductus arteriosus.

the left innominate vein. The right duct of Cuvier and the right jugular, below the entrance of this cross branch, form the superior vena cava; while the part of the right primitive jugular

immediately above the entry of the left innominate vein forms the right innominate vein.

The posterior vertebral vein of the right side forms the vena azygos major; the corresponding branch of the opposite side, together with the part of the left primitive jugular below the cross branch, forms the left superior intercostal vein and the superior vena azygos minor. The lower part of the left posterior vertebral vein, together with the connecting branch to the right vein, remain as the inferior vena azygos minor.

Fatal Circulation.-The course taken by the blood through the heart and vessels of the embryo differs essentially from that which persists in adult life.

Tracing the blood from the placenta, it passes along the umbilical vein toward the liver; here it may take either of two courses to reach the vena cava, one which follows the ductus venosus and avoids the liver, the other which passes by the venæ advehentes (portal veins) to the liver, and proceeds by the venæ revehentes (hepatic veins) to the inferior vena cava, which receives all the blood passing by both of these channels. From this the blood is emptied into the right auricle, and hence is guided by the Eustachian valve through the septum by the patent foramen ovale to the left auricle. From the left auricle it passes to the left ventricle, which contracts and sends the blood into the aortic arch, where it is split up into two streams, one of which passes into the vessels of the head and neck, the other by the descending aorta to the trunk and lower extremities.

The blood from the head and neck is returned to the right auricle by the superior vena cava. The blood from this vein passes through the auricle to the right ventricle, which sends it through the pulmonary artery toward the lungs.

The pulmonary artery, in the embryo, has one very large branch, called the ductus arteriosus, which joins the aorta at a point just below the origin of the vessels of the head and neck; hence the main part of the blood passing from the right ventricle reaches the aorta by the ductus arteriosus, and only a very small part goes to the lungs, to be returned from them by the pulmonary veins to the left auricle.

The blood from the ductus arteriosus blends, therefore, with that in the aorta which is passing to the viscera and lower extremities. The main part of this blood travels by two large branches of the aorta (the hypogastric arteries) to the placenta, where it is aerated and purified, etc.

It is evident, if the placenta is the great renovating organ of the blood of the foetus, that the blood in the umbilical vein is the most arterial in the foetal circulation. The blood in the ascending vena cava and first part of the aorta is likewise fairly arterial, but the blood in the descending aorta is of a mixed character, as it contains blood which has nourished the head and neck, besides that which has come from the placenta by the inferior vena cava through the right auricle, foramen ovale, left auricle, and left ventricle.

As the foetal lungs are not called into play until after birth, but little blood passes to them in the fœtus; this state of things is, however, completely altered at birth, when the lungs of the child expand, the pulmonary arteries increase in size, and the ductus arteriosus dwindles in a corresponding degree.

The liver, which in the foetus is of relatively greater size than in the adult, receives much blood coming from the placenta to the heart, and is thought to contribute to it several essential constituents.

The head and brain, which are largely developed in the fœtus, receive well aerated blood; namely, the placental blood which has passed through the liver, and, in the inferior vena cava, is mixed with blood coming from the lower limbs. The rest of the foetus receives blood that is less aerated, as it is mixed with that which is returned from the head and neck to the right side of the heart, and which is sent through the ductus arteriosus to join the general blood current in the aorta going to the viscera and lower extremities.

DEVELOPMENT OF THE EYE.

The optic vesicles arise from the anterior cerebral vesicle at a very early period, and their cavities are continuous with that of the fore-brain. With the development of the rudimentary cere

bral hemispheres the optic vesicles become displaced downward, and their cavities open into the junction of the cavities of the cerebral hemispheres, and that of the thalamencephalon, which becomes the third ventricle. Later, the optic vesicles open directly into the third ventricle, and finally are displaced backward, and come into connection with the mid-brain.

The optic vesicles are at first hollow prolongations, which consist of an anterior dilated portion, forming the primary optic vesicle, and a posterior tubular portion or stalk joining the vesicle to the fore-brain. This stalk forms the optic nerve.

As each vesicle grows forward toward the epiblast covering the head of the embryo, the epiblastic cells at the spot overlying the vesicle become thickened, an involution of the epiblast takes place toward the optic vesicle, and indents the latter, approximating its anterior to its posterior wall.

FIG. 310.

a

Section through the head of a
chick at the third day, show-
ing the origin of the lens.
a. Epiblast thickened at,
which is the point of origin
of the lens. o. Optical vesi-

By this means the anterior and posterior walls of the primary optic vesicle come into close contact, and the cavity of the vesicle is obliterated. The two layers of the vesicle are now cup-shaped, and receive the name of the secondary optic vesicle or the optic cup. This ultimately becomes the retina, and the optic stalk, losing its cavity, is transformed into the optic nerve.

Meanwhile, the local involution of the epiblast over the optic cup, which is the rudiment of the crystalline lens, becomes gradually separated from the general epiblast, and is finally detached from its point of origin. It now lies as a somewhat spherical body in the cavity of the optic cup within the superficial mesoblast, which has closed

bral vesicle.

over it.

The secondary optic vesicle grows (except at its lower part, just at the junction of the optic stalk) so as to deepen the optic

cup, which contains the rudimentary lens. At the lower part an interval is left, which receives the name of the choroid fissure. Through this gap in the secondary optic vesicle the mesoblast

h

D

FIG. 311.

E

bon

Diagrammatic sections of the primitive eye, showing the choroidal fissure. (Foster and

Balfour.)

D. Horizontal section. E. Vertical transverse section just striking the posterior part of the lens. F. Vertical longitudinal section through the optic stalk, and the fissure through which the mesoblast passes to form the vitreous humor.

k. Superficial epiblast. r. Point of origin of the lens. v. k. Vitreous humor. r. Anterior layer of the optic vesicle. u. Posterior layer of the optic vesicle. c. Cavity of the optic vesicle. Choroidal fissure. s. Optic stalk. . Cavity of the optic stalk. 7. Lens. l. Cavity of the lens.

enters and separates the lens from the optic cup, forming the vitreous humor.

The mesoblast surrounding the optic cup develops two cover

ings of the eye, an outer fibrous capsule called the sclerotic coat, and a vascular coat, the choroid.

In front of the lens, beneath the epiblast, the mesoblast forms