574 THE HUMAN EMBRYO. 



into the two large allantoic arteries, which carry blood from the 

 embryo to the placenta. 



The aortic arches of man, as of other Vertebrates, are de- 

 veloped in order from before backwards. 



At the fifteenth day (Figs. 197 and 232) there are two pairs 

 of aortic arches present, lying in the mandibular and hyoideanj 

 arches, and corresponding, therefore, to the most anterior 

 pairs in rabbit, chick, or frog embryos. By the sixteenth day 

 three additional pairs have appeared, in the first, second, and 

 third branchial arches ; and up to the end of the third week all 

 five pairs are still present (Fig. 198, A.1-A.5). 



The point of attachment of the truncus arteriosus to the 

 floor of the mouth shifts backwards during development, as 

 already noticed, and at the end of the third week is opposite the 

 hyoidean and first branchial arches. The truncus arteriosus, at 

 this stage, immediately on entering the floor of the mouth, divides 

 into two branches on each side (Fig. 198). Of these, the an- 

 terior branch runs forwards, and divides into the mandibular, A.i, 

 and hyoidean, a. 2, aortic arches ; while the posterior branch runs 

 backwards, and divides into the three hinder aortic arches, A.3, 



A.4, A.5. 



The aortic arches diminish in size from before backwards 

 (Fig. 198) ; and, owing to the funnel-like shape of the pharynx 

 (cf. Fig. 238), the hinder arches lie much nearer the median plane 

 than do those further forward. 



All five pairs of arches are complete, opening at their dorsal 

 ends into the aortas (Fig. 198). In front of the first, or mandi- 

 bular arch, each aorta is continued forwards as the internal 

 carotid artery, which runs along the side of the brain, and gives 

 off branches supplying this. 



During the fourth week important changes occur in the 

 aortic arches, closely comparable with those already described in 

 other Vertebrates, and leading to the establishment of the adult 

 scheme of circulation. 



Early in the fourth week (Fig. 243) the middle portion of 

 the first, or mandibular, aortic arch of each side becomes obli- 

 terated, and disappears ; and very shortly afterwards the cor- 

 responding portion of the second, or hyoidean, aortic arch 

 disappears in the same fashion. 



By the end of the fourth week the condition of the aortic 



