507 



in human, rabbit, and sheep embryos. The aortic arches may be 

 defined as vessels extending from the ventral to the dorsal aorta and 

 found either between the successive pharyngeal pouches, in front of 

 the first, or behind the last. The arch discovered by Zimmermann 

 in the rabbit and sheep accorded with this definition, but that in man 

 was modified inasmuch as it arose from, and terminated in, the fourth 

 arch. In 1902, Tandler ^) published excellent reconstructions of these 

 arches in man and in the rat. His two human embryos which pres- 

 ented the new arch differed from that described by Zimmermann, since 

 the arch arose from the ventral aorta, near the fourth however, and 

 emptied into the pulmonary arch at the beginning of its distal third. 

 In the rat, Tandler found an anastomosis, made by a very short 

 vessel, connecting the fourth and pulmonary arches about midway in 

 their course. He wrote: — "If I would homologize this connection 

 with the fifth aortic arch in the rabbit and in man, I might do so 

 upon the following grounds. I found in spite of the careful exami- 

 nation of a whole series of successive stages no vessel other than this 

 which could be compared with a fifth arch. Since we must postulate 

 in the rat, as well as in all other mammals, a fifth arch, I consider 

 myself justified in regarding this connection as the analog of the fifth 

 arch, and so much the more since the fifth arch appears to vary in 

 different species, and even in difi'erent embryos, as in man, both in 

 regard to its origin and its course . . . Moreover I may note that I 

 have not succeeded, at least in the rat, in determining an entodermal 

 outpocketing which can correspond with a fifth pouch" 2). 



In a brief account of the development of the aortic arches in 

 rabbits, presented by the writer to the American Association of Ana- 

 tomists, in 1903, it was stated that "the irregular small arteries 

 around the fourth entodermal pouch do not, as Zimmermann believed, 

 form a distinct aortic arch" ^). 



In 1905, Miss Lehmann pictured the aortic arches in the rabbit 

 and pig. Of the rabbit she writes^) — "In the space between the 

 fourth and sixth arches there are structures which appear to be ele- 

 ments of an incomplete arch. , . These elements do not fully agree 

 with Zimmermann's description of a fifth arch for the rabbit. He found 



1) J. Tandler, Morph. Jahrb., Bd. 30, 1902, p. 275—373. 



2) J. Tandler, 1. c. p. 340—341. 



3) F. T. Lewis, Americ. Journ. of Anat., Vol. 3, 1904, p. XIIL 



4) H. Lehmann, Anat. Anz., Bd. 26, 1905, p. 412. Similar state- 

 ments are found on pages 405 — 407 of Miss Lehmann's later paper, 

 Zool. Jahrb., Abt. f. Anat., Bd. 22, 1905, p. 387—434. 



