THE VASCULAR SYSTEM 331 



the circulatory, but in other systems as well, is pushed back 

 among the stages that are recapitulated in the embryo; there 

 is a metamorphosis in reptiles and mammals just as truly as 

 in the case of amphibians, but it is embryonic. Here, as in 

 amphibians, arch III, with rudiments of I and II, forms the 

 carotids and its connection with arch IV disappears. This 

 latter becomes the aortic arch, and is retained on each side, 

 as right and left arches, the two uniting dorsally and back of 

 the heart to form the main aorta. Arch V, which in am- 

 phibians is practically superfluous, is given up in reptiles, 

 and from this point on is seen no more, save in the em- 

 bryo, where it often appears as a rudiment. A pulmonary 

 artery develops from the sixth arch of each side, as in am- 

 phibians, leaving a right and left ligamentum arteriosum [Bo- 

 talli]. The subclavian arteries, that supply the fore-limbs, 

 which in most fishes and amphibians arise from the dorsal 

 aorta after the union of the two lateral arches, possess a mor& 

 anterior origin and arise from the right aortic arch. Croco- 

 diles and turtles present an exception to this, and in these, 

 as in birds, the subclavians arise from the base of the carotids, 

 an origin so radically different as to lead morphologists to be- 

 lieve that these vessels are not the subclavians at all, but are 

 secondarily developed arteries (subclavia secundaria) which 

 have functionally replaced the true subclavians. 



In birds and mammals (Fig. 91, e and f) but a single 

 aortic arch comes to development ; in birds this is the one on 

 the right side, in mammals the one on the left, a convincing 

 proof, if proof were wanting, of the independent development 

 of these two Classes. There is thus,^in each case, but one 

 ligamentwm arteriosum, connecting the pulmonary and aortic 

 arches. In the mammalian fetus, in which pulmonary respira- 

 tion is not assumed till the moment of birth, this vessel is 

 functional and is known as the ductus arteriosus [Botalli]. 

 It is still pervious at birth, but the lumen closes within a few 

 days by the rapid thickening of the wall of the vessel. There 

 is here to be noted an important difference also in the sub- 

 clavians; in birds, as i^ the turtles, these vessels are repre- 



