AORTIC-ARCH SYSTEM IN THE HUMAN EMBRYO. 57 



(1915a), in his study of the development of the human pharynx, points out that the 

 nature of the components of the caudal pharyngeal-pouch complex, exclusive of the 

 fourth pouch, is still too uncertain to justify the claim of a fifth visceral arch. 

 He finds, however, that in the human embryo possible fifth pouches may reach the 

 integument. Whether they are rudimentary fifth arches or not, there seems to be 

 warrant for considering these structures as more homogeneous and definite in 

 character in man than has been generally recognized. The residue left after the 

 islands are eliminated consists, for the most part, of channels passing from near 

 the dorsal end of the pulmonary arch to the proximal end of the fourth arch or the 

 adjacent aortic sac. The chief variation from this type is offered by vessels that 

 terminate distally in the fourth arch. The sprouts lying in grooves of the caudal 

 pharyngeal complex and otherwise having the same relations as the ends of these 

 channels may be regarded as incomplete stages of the same type. Their frequency, 

 taken with that of the complete channels, was found to exceed 50 per cent. 



The so-called fifth arch is described by several authors as arising later than the 

 pulmonary. In the human embryo, at least, it will require further data to deter- 

 mine the time relation between the two vessels. The difficulty lies in the lack of a 

 precise period at which we may regard an arch as coming into existence, owing to 

 the gradual nature of its development from a plexus. Nothing is known of the 



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CL 



Fig. 17. Development of the pulmonary artery and ductus arteriosus, showing degeneration of distal part of right 

 arch and the incorporation of its proximal part into right branch of pulmonary artery; also approach 

 of right and left branches through wall of pulmonary stem, a, 7-mm. embryo, No. 617; b, 11-mm. 

 embryo, No. 1121; c, 13-mm. embryo, No. 1771; d, lS-mm. embryo, No. 1390; e, 43-mm. embryo, 

 No. 886. 



manner in which the so-called fifth arch disappears. Certainly it does not retain 

 its individuality long, since it has not been described in older mammalian embryos. 

 As one follows the deep-seated changes of the parts of the arch system from which 

 the aortic arch and pulmonary artery are formed, it becomes easy to picture its 

 early interruption and the taking up of more or less of the material of its wall in 

 these larger vessels. It may be that some of the spurs which have been described 

 in this region are stages in the development, while others are steps in the regression, 

 of the so-called fifth arches, and it is very likely that the transition from the former 

 to the latter is frequently accomplished without the establishment of a complete 

 channel. 



Shaner (1921) states that in vertebrates it is not rare for the sixth arch to 

 develop, after the fifth is established, as a shorter vessel coming off from both ends 

 of the fifth. The intermediate segment of the fifth then disappears, leaving its 



