2o8 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 aortse 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 impoi-tant modifications of 

 the aortic arches will j^erhaps 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 ai'teries 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 oi'gan. 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, Avhen 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 follous .• — 



The apj^earance 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 secondar}^ optic vesicle. 



First appearance of the nasal pits. 



