208 EMBRYOLOGY 
of the atmosphere; it returns through the vena terminalis into 
the heart as oxidized or arterialized blood. Besides this complete 
circulation of the yolk-sac, the body of the embryo itself has received 
a circulation. A pair of anterior and posterior cardinal veins collect 
the blood from the body, and convey it through a right and a left 
Cuvierian duct into the heart. The two primitive aorté are united 
into one median dorsal aorta, but in the region of the neck, instead of 
the single right and left aortic stems, several aortic arches appear—six 
on each side, although not more than three or four are present at 
the same time. From them are sent off the carotid arteries into 
the head ; these and other subsequent important modifications of 
the aortic arches will perhaps best be understood by reference to 
the accompanying diagrams. ‘The first, second, and fifth transverse 
arches obliterate very early ; the third pair is continued along the 
neck and into the head as the internal and external CAROTIDS 
and also sends off the subclavian arteries for the anterior extremities. 
The fourth arch of the right side is transformed into the ascending 
arch of the big aorta, while the corresponding parts of the left side 
disappear. ‘The last or sixth pair is transformed into the pulmonary 
arteries; the connexion of the right pulmonary with the aortic 
trunk remains for some time as the “ductus Botalli.” Simul- 
taneously with these changes goes the transformation of the simple 
tubular heart into a four-chambered organ. The heart-tube assumes 
an S-shaped twist; a septum begins to grow out from the inner 
wall, and indicates the division of the bulged-out middle portion of 
the heart into a right and left ventricle ; and to complete this part 
of the subject it may be added that this division is completed on 
the fifth day, when a similar septum separates the posterior or 
venous portion of the heart into a right and left atrium, each with 
a lateral dilatation or auricle. ‘This atrial septum is not completed 
before the twelfth day, the right and left atrium communicating. 
with each other until this time by the “foramen ovale.” On the 
fifth day a longitudinal continuation of the ventricular septum into 
the anterior or arterial portion of the heart and into the root of the 
ventral aorta divides this bulbus arteriosus into a truncus arteriosus 
and a truncus pulmonalis. As the lungs are being formed, pul- 
monary veins also make their appearance, and become connected 
with the left atrium of the heart. By the end of the fifth day 
most of the principal arteries and veins of the body have likewise 
been developed. 
The remaining changes on the 3rd day are as follows :— 
The appearance of the vesicles of the cerebral hemispheres ; the 
separation of the hindbrain into cerebellum and medulla oblongata. 
The lens of the eye is formed by involution of the optic vesicle, 
and the eyeball appears as the secondary optic vesicle, 
First appearance of the nasal pits. 
