= 266 
that the pulmonary artery arises from the sixth aortic arch in Cera- 
todus, Polypterus, Amia, and, that in ampibia, there is a rudimentary 
arch between the systemic and pulmonic which, in rare cases persists 
in adult stages (Salamandra), but is usually completely transitory (frog, 
Triton, other forms and frequently in Salamandra). The point of espe- 
cial interest in the combined observations of Boas and Van BEMMELEN, 
was in showing that the pulmonary artery arises from the sixth aortic 
arch in amphibia, reptiles and birds, instead of from the fifth as pre- 
viously supposed. And, the question was at once opened as to whether 
the posterior arch in mammals is the fifth or the sixth? The determin- 
ation of this point is quite essential for the comparative anatomy of 
the arterial system, since it has been shown that in all the air-breathing 
vertebrates the third and fourth arches persist as the carotid and sys- 
temic, respectively, and that the pulmonary artery arises from the 
posterior arch. 
After the publication of Van BEMmMELEN’s paper Boas reviewed 
(1887) the evidence and suggested the probability that a fifth rudimen- 
tary arch had been overlooked in mammals, and he predicted that such 
an arch would be found. He avowed his belief that the pulmonary 
artery arises from corresponding arches in mammals and other verte- 
brates, and that the number of aortic arches exhibited in the embryonic 
history of all vertebrates above the fishes is six. He prepared diagrams 
on this assumption which have become standard ones in text-books. 
The prediction of Boas apparently came true in the work of 
ZIMMERMANN who, in 1889, demonstrated before the Anatomische Ge- 
sellschaft a reconstruction of a human embryo 7 mm long showing a 
hitherto unobserved condition of the aortic arches among mammals. 
This consisted of an offshoot from the systemic arch, lying between 
the latter and the pulmonic, in the form of an additional aortic arch. 
This newly observed artery was described as slightly smaller than the 
fourth arch and given off between the first (lower) and second-thirds 
of that arch. It is at first directed posteriorly, then, taking a course 
nearly parallel to the arch passes upward and unites again with the 
fourth arch between its middle and last thirds. In the same year 
ZIMMERMANN described a complete arch between the systemic and pul- 
monic in a rabbit of about the eleventh day of development. The 
notable circumstance in his specimen of the rabbit is that the arch 
extends from the truncus arteriosus to the aortic root without being 
connected with either the systemic or pulmonic arches. It leaves the 
truncus arteriosus as a moderately slender vessel, broadens gradually, 
until it becomes about one-half the diameter of the pulmonary arch and 
empties into the aorta close to the distal end of the latter arch, 
ZIMMERMANN found in both human and rabbit embryos an epithelial 
evagination lying between the new arch and the others. He also found 
in an incomplete series of sections of a sheep embryo a corresponding 
vessel present on both sides of the pharynx leaving the distal end of 
the pulmonic arch and extending ventralward between the systemic and 
pulmonic arches. Owing to the incompletness of his sections he was 
not able to determine its ventral termination. From these observations 
