DEVELOPMENT OF TEE FECUNDATED EGG. 517 



alone remaining to form the large and permanent branches 

 of the circulation. In this way the highest of these arches 

 becomes the right brachio-cephalic (arteria innominata), the 

 carotid, and the left subclavian arteries 

 (Fig. 150, 5, 4) ; on the right side the 

 second arch disappears, but, on the left, it 

 forms the arch of the permanent aorta (3) ; 

 the third sends off on each side a branch 

 which ramifies through the lung of the 

 corresponding side; while, on the right, 

 the portion beyond this offshoot becomes 

 atrophied, and its companion on the left 

 side remains and furnishes a communica- 

 tion between the pulmonary artery and 

 the arch of the aorta; this is called the Fig. iso Aortic arches 

 ductus arteriosus. This ductus arteriosus J gj J^g n # ent arte " 

 holds the same relation to the placental 

 circulation as the foramen ovale (of Botal) and the ductus 

 venosus (of Arantius) (see p. 515). 



The bulb of the aorta, moreover, is so divided that the part 

 of its cavity, which communicates with the left ventricle, 

 is a continuation of the remains of the two first pairs of 

 aortic arches (carotids, subclavians, and arch of the perma- 

 nent aorta) ; whilst that part of its cavity, which communi- 

 cates with the right ventricle, is a continuation of the 

 remains of the third aortic arch, the pulmonary artery (and 

 the ductus arteriosus) (Fig. 150, 1). 



If we pursue the arrangement of the arterial system from 

 centre to the periphery, we shall find that the descending 

 portion of the aorta gradually elongates (p. 512), and that 

 the two posterior vertebral arteries become the iliac arteries / 

 from these latter are given off two, relatively, very large 

 branches, which are called the umbilical arteries ; these fol- 

 lowing the pedicle of the allantois, and entwining around the 

 single umbilical vein in the umbilical cord, convey blood 

 from the foetus towards the placenta; at this point the blood 

 is distributed in the capillaries of the villi, and comes into 



* 1, Trunks which spring from each ventricle (bulbus arteriosus dividing 

 into the origin of the aorta and origin of the pulmonary artery ); above, live 

 pairs of aortic arches may be seen ; the highest of these disappear ; only the 

 three which are nearest the heart become permanent vessels, and represent the 

 subclavian, the right, and the left carotid arteries 5, 4. 3, The arch of the aorta. 

 2, The descending portion of the aorta ; the ductus arteriosus, which has only a 

 very transitory existence, may be seen at the junction of the arch of the aorta 

 with its descending portion. 



