186 GEO. S. HUNTINGTON 
also exists (pinnipedia, perissodactyls, camelidae, aquatic ro- 
dents, arboreal primates, Hippopotamus). The early onto- 
genetic stages must therefore offer opportunities which corre- 
spond to the above findings in the adult. The ventral wall of 
the foregut with the vagi forms in rabbit embryos of 10 mm. 
to 11 mm. the background supporting the tracheal bifurcation 
and the stembronchi with their primary buds. The pulmonary 
arteries accede to the ventral-lateral circumference of the trachea 
along which they at first descend in a fairly symmetrical course. 
In approaching the tracheal bifurcation the right artery gradually 
inclines ventrad, while the corresponding vessel of the left side 
turns dorso-caudad. This is the result of the rotation of the 
heart and its arterial pedicle to the left. At the same time the 
foregut carrying the two vagus trunks undergoes the axial rota- 
tion to the right through which the left side of the gastric 
enlargement attains its direction ventrad. 
As the result of these rotations of heart and foregut in opposite 
directions the topographical conditions are changed on the two - 
sides. The left vagus and left pulmonary artery gradually 
approach each other and are, at the level of the tracheal bifurca- 
tion and origin of the stembronchi, in close contact. The reverse 
obtains on the right side. The right pulmonary artery turns 
more and more ventrad in descending, while the right vagus, 
following the rotation of the foregut, moves dorsad. An arterio- 
neural interval is thus opened up on the right side toward which 
the latero-dorsal circumference of the right stembronchus faces 
directly and into which it sends the right eparterial bronchial 
bud. The latter thus comes to lie in front of the right vagus 
and right side of the oesophagus, and behind the right plumonary 
artery. On the left side this arterio-neural portal for eparterial 
development has been blocked by the approximation of left 
vagus and left pulmonary artery, or reduced to such an extent 
that it no longer affords a favorable path for the protrusion of 
an eparterial bud from the left stembronchus. The first pri- 
mary derivative of the latter is hence forced to pass ventral to 
the artery and thus becomes the first ventral hyparterial bron- 
chus of the left side. This supplies through its large, ascending 
