10 C. W. M. Poynter 
monic arch, Bremer (1912) says: “In the strictest sense the arch 
extends only from the dorsal aorta to the pulmonary artery; the 
ventral part of the vessel usually called the arch is really the 
ventral aorta.” 
The development of the pulmonary arteries is readily under- 
stood by reference to Bremer’s work (1902-6) which shows that 
Rathke’s error was due to a failure to study the early develop- 
ment of the vessels, the condition which he figured being a later 
stage; see figs. 3 and 5. The arteries in the earlier stages de- 
velop one on each side and shift to the left with the growth and 
torsion of the truncus pulmonis about the bulbus aortae. 
The subclavian arteries already referred to, fig. 3, Rathke 
(1857) considered arose as figured and arrived at the adult con- 
dition by a shift of the arch and a coalescence of the roots of the 
subclavian and carotid. This explanation was accepted till 1888 
when Mackay presented a study of the subclavians in the chick. 
This was followed by the work of Hochstetter (1890), Goeppert 
(1908) and Evans (1909) for mammals, which established the 
origin of these arteries from a number of segmental arteries, vari- 
able in number, from the dorsal aorta. This work agrees with 
or explains the variations of the subclavians so frequently en- 
countered. Figs. 5 and 8 illustrate the possibilities of origin of 
these arteries. 
We are indebted to Hochstetter (1890) and to its further 
elaboration by Kemmetmuller (1911) for the correct interpreta- 
tion of the origin of the vertebral arteries. Fig. 6 illustrates 
diagrammatically the origin of these arteries and their possible 
variations. 
I have reviewed the embryonal history of these vessels in order 
to establish a basis for classification and explanation of the varia- 
tions which are to follow. While we are not directly interested 
in the earliest stages of the bloodvessels, all of the recent work 
has shown that many vessels are preceded by plexus formation, 
though not to the extent suggested by Baader, and this fact may 
account in a few cases for the formation of unusual anomalies 
which do not seem to belong to the general classifications. 
228 
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