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been shown in chick embryos, it may not be out of place to review 
briefly the observations on other vertebrates, with a view to estimating 
the bearing of these observations upon the question of the existence 
of six aortic arches in mammals. 
Leaving out of consideration all fishes except the Dipnoi, in 
which there is a functional lung, it is established that in Ceratodus 
there are six aortic arches, the pulmonary artery arising from the 
sixth. In Protopterus and Lepidosiren there are also six arches but 
the pulmonary artery comes from the aortic root. 
In amphibia variable conditions are exhibited. In Salamandra, 
the fifth arch sometimes persists in the adult but usually disappears. 
It shows much individual variation. The original figure by Boas of the 
arterial arches of an adult Salamandra, which is executed in lines, 
shows the fifth arch arising independently from the truncus arteriosus 
and extending to the aortic root. in the copy of that figure, so widely 
known through WIEDERSHEIM’s Comparative Anatomy of Vertebrates, the 
drawing is executed in black and the fifth and sixth arches appear to 
blend, which gives the impression that the fifth is an offshoot of the 
sixth, but this is evidently not the case as shown in the drawing of 
Boas. 
In the Frog the fifth arch is also extremely variable. It is fre- 
quently entirely absent, but, in those cases in which its presence has 
been shown by Boas, it arises as 
ee an offshoot of the sixth arch and 
i N empties into the aortic root. Fig. 6 
ann is a copy of one of his figures 
I ar. — showing the fifth arch in a young 
wo eX \ frog which has just lost its tail. 
‘ : The position of the pulmonary 
Fig. 6. Aortic arches of the left 
side in a young frog (after BOAS). «ao 
aortic root; ce external carotid; ci in- 
ternal carotid; cw cutaneous branch; p 
pulmonary artery. 
artery near the distal extremity of 
the arch is to be noticed. 
In Cryptobranchus, as the 
figure of R6se shows, the fifth 
arch arises from the truncus ar- 
teriosus and has an independent connection with the aortic root. The 
presence of the fifth arch has been shown in Triton and other am- 
phibia. Besides the work of Boas that of MAURER should be meritioned 
in this connection. 
The existence of six aortic arches in reptiles has been shown also 
by a considerable number of observers — 
Boas, VAN BEMMELEN, 
HOCHSTETTER, PETER and others. VAN BEMMELEN’S figure of a recon- 
