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complete arch as an offshoot of the fourth arch and returning to it 
(human, ZIMMERMANN); b) a complete arch connecting the fourth 
and pulmonic arches (pig, LEHMANN, rat, probably, TANDLER); c) a 
complete arch from the truncus arteriosus to the aortic root (rabbit, 
ZIMMERMANN); d) a complete arch from the truncus arteriosus to the 
pulmonic arch (human, two cases, TANDLER); e) irregular vascular 
elements from the fourth arch, the aortic root and the pulmonary 
arch (pig, LEHMANN, Lewis, rabbit, Lewis, LEHMANN, rabbit in stages 
of degeneration of the arch, ZIMMERMANN) and f) a single vessel from 
the distal end of the pulmonic arch with unknown ventral connections 
(sheep, TANDLER). 
It would appear from the foregoing that the diagramatic represent- | 
ations of the fifth arch as a division of the fourth (cf. Mc Murricn, 
The Development of the Human Body, Fig. 137) are likely not typical 
even for the human embryo. TANDLER shows it in two cases in the 
human embryo connected with the sixth arch, and the other observ- 
ations so far made, show that, in the vertebrate series, it is more 
frequently connected with the pulmonic than with the systemic arch. 
In the light of all these observations, it seems clear to the 
writer, that the vascular elements exhibit great variability in their stages 
of formation and degeneration, but, that there are recorded a sufficient 
number of cases of their aggregation into a complete vessel, to justify 
the assumption that there is a fifth aortic arch in mammals as in other 
vertebrates. 
The variations and irregularities should be taken into account but 
they should not dominate interpretations. The ever changing picture 
in embryonic development must be constantly borne in mind. One set 
of sections, at best, represents only one stage either of construction 
or obliteration of such transitory structures as the aortic arches, and, 
therefore, those cases in which they are most perfectly exhibited are 
of more significance than those in which they are partially or imper- 
fectly represented. 
Variability of transitory organs is also a constant feature of 
embryonic development. The vascular system even in adult stages is 
subject to wide variation, and in the embryo, individual variation in 
the aortic arches, and all their dependences, are commonly met with. 
In the fifth arch we have a structure of unusually brief existence, 
and great variation in its development is to be expected. It is likely 
that the drawings of pig and rabbit embryos, showing irregular arterial 
rudiments between the fourth and pulmonic arches, represent stages 
in which that arch is incomplete, or individual specimens in which 
