EXPLANATION OF FIGURES 
Fic. 1. This figure represents the fig. 3, Plate II, used by v. Baer to 
illustrate what he, at that time, thought was the type of development of 
the aortic arches in mammals. It was copied by Thomson (1831) as fig. 
39, although he did not give v. Baer credit for the mammalian figure but 
for the right arch figure for birds, his fig. 30, and claimed priority for the 
presentation of the mammalian type. 
Fic. 2. This figure represents the fig. 14, Plate lV, which v. Baer (1837) 
used to illustrate his embryology. He offered this figure in place of the 
one above to correct certain misconceptions of the fate of the aortic arches 
in mammals. He was of the opinion that the primitive third arch on each 
side forms the arterial stem which divides into the internal carotid sub- 
clavian and vertebral arteries, and that the external carotids arise from 
the low persisting portion of the two first arches of one half side of the 
head. 
Fic. 3. This figure represents fig. 10, Plate VI, offered by Rathke (1857) 
to correct the errors in interpretation which he considered v. Baer had 
made for mammals. The development of the carotids and subclavians in 
the main is correct and the relation of the pulmonary arteries to the fifth 
arch is for the first time presented. This figure has frequently been pre- 
sented as if it represented the first attempt at interpreting the arches, 
when in fact it) is merely one step in the progress of our understanding 
of these structures. 
Fic. 4. This figure represents fig. 1, Plate I, offered by Boas (1888) to 
illustrate the development of the aortic arches in mammals and especially 
to introduce an additional arch between the fourth and pulmonary arches 
which he believed had been overlooked by previous observers. 
Fic. 5. This figure represents our present understanding of the devel- 
opment of the aortic arches in mammals and is used in this study as the 
basis for modifications explaining the various anomalies encountered in 
this work. 
Fic. 6. This figure is adapted from Kemmetmiiller (1911) to illustrate 
the development of the vertebral arteries and the possible variations they 
may assume due to anomalous development. 
Fic. 7. This figure is adapted from Banchi (1907), his fig. 7, used to 
illustrate the method of development of the low subclavian, taking a pre- 
tracheal course. This method of solid reproduction of the arches was 
used by His, and Banchi urges that it is much easier by this type of figure 
to present the actual condition of the arches than by the plane figure, as 
above. 
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