56 AORTIC-ARCH SYSTEM IN THE HUMAN EMBRYO. 



of the arches to elongate is due to the lack of active growth in their immediate 

 environment (the caudal portion of the pharynx) at this time, and this in turn is an 

 expression of the regressive changes which the organ undergoes. 



The chief cause of the disappearance of the first and second arches is probably 

 to be found in the shift of the blood-stream to the more caudal arches, which ac- 

 companies the caudal movement of the aortic sac. The rapid growth of the 

 propharynx, 3 in both width and length, doubtless hastens their degeneration by 

 increasing the length of their course. 



SO-CALLED FIFTH ARCH MORPHOLOGY OF PULMONARY ARCH. 



There are two vascular types that appear in descriptions of the so-called fifth 

 aortic arch in mammals, and both occur frequently in man. One is the island- 

 formation of the upper end of either the pulmonary or fourth arch, the other is a 

 channel connecting the fourth and pulmonary arches. Most frequently this 

 vessel comes from the proximal end of the fourth arch, or the subjacent aortic sac, 

 and enters the pulmonary arch above. Its upper end sometimes enters the fourth 

 arch. It may be represented only by spurs corresponding to its extremities. 

 The islands at the upper end of all of the arches but the first and at the lower 

 extremity of some of them have already been referred to and interpreted as retained 

 parts of the plexus which precedes the arches (fig. 4). They require no considera- 

 tion in a discussion of the fifth arch. 



Models of 7 embryos in which the pulmonary arch was almost or just com- 

 pleted were available. Among them were found 3 well-developed vessels arising 

 from the aortic sac or fourth arch and ending above in the distal end of the pul- 

 monary arch (figs. 8, 18, 22). One was of much smaller diameter than the arches, 

 but another was as large as the fourth arch. They all lay in deep grooves of the 

 caudal pharyngeal complex. Arterial sprouts corresponding to the ends of these 

 vessels were found in relation with many of the other caudal pharyngeal complexes 

 and usually can be shown to He in corresponding though more shallow grooves. 

 The propriety of regarding these channels as rudimentary fifth arches is still a 

 matter of debate after the passage of nearly forty years since Van Bemmelen 

 (1886) claimed their existence in mammals and in spite of the work of nearly a score 

 of investigators. Tandler (1909) was the first to describe them for man, and figured 

 vessels similar to those observed in our series, except that they had a somewhat 

 longer dorsoventral course. He also found spurs corresponding to their ends. 

 He believed that these constitute true fifth aortic arches, but regarded them as 

 very transitory. Only 6 instances of the complete vessels in man have been de- 

 scribed up to this time. More than 20 have been found among the lemur, mole, 

 rabbit, cat, guinea-pig, and pig. 



It was a corollary to the principle that embryonic blood-vessels depend greatly 

 upon their environment for their form that Lewis (1906), in a study of rabbit and 

 pig embryos, denied the authenticity of so-called fifth aortic arches, on the ground 

 that the existence of fifth visceral arches had never been proved. Kingsbury 



3 Kingsbury distinguishes the cranial portion of the pharynx, including the second visceral arch, by this term, and calls 

 the more caudal part the metapharynx. The propharynx grows more rapidly in length and width than the caudal division. 



