Joseph Marshall Flint 73 
point of origin of Lateral 1, the tracheal bronchus. This relationship 
and the subsequent behavior of the two arches as the heart moves down 
affords us, I believe, some suggestive hints to explain the suppression 
of Lateral 1 on the left side and its unusual low position in those ani- 
mals in which it is present. Through all of the stages we have followed 
hitherto, both the aortic and pulmonary arches, and the origin of the pul- 
monary arteries lie well above the origin of Lateral 1. As shown by a 
corrosion of the bronchi, arteries and veins in an embryo 15 cm. long, 
the pulmonary arch is exactly opposite the site of origin of Lateral 1, 
while the aortic arch is still higher. At the age represented by a pig 
20-21 cm. long, the heart and vessels have descended further caudal- 
wards, leaving the pulmonary arch well below the root of Lateral 1 and 
the aortic arch exactly at its level. At the time of birth, both arches 
have descended still more and pass dorsalwards in the interval between 
the trachea, the stem bronchus, and the apical branch of Lateral 2 
(cf. Pl. IV, Fig. 25). Now, had a symmetrical branch to Lateral 1 
developed on the trachea, it is obvious that the descent of the great vessels 
and heart would have been prevented. Instead of reaching their final 
resting place just above the division of the trachea, they would have 
been left hanging above the level of Lateral 1. It is thus reasonable to 
suppose that the failure of this branch to form is due to a phylogenetic 
provision on the part of the tree to leave a passage for the descent of 
the heart and its great vessels. 
A similar state of affairs is met with in the suppression of Ventral 2 
on the left side. As the pulmonary vein forms approximately in the 
median line in the younger stages, the hyperdevelopment of right Ventral 
2, the development of the inferior vena cava on the right side, and the 
shifting of the origin of the pulmonary vein from the site of its forma- 
tion near the center of the undivided portion of the auricle to the left 
auricle, together with the increasing asymmetry of the heart, tends to 
carry the vein to the left. From its primitive approximate midline posi- 
tion in the earlier embryos, it is found with the increasing age of the 
embryo gradually passing to the left. In a pig 15 cm. long, we have 
the vein for the entire lower segment of the tree lying over the portion 
of the stem where left Ventral 2 should develop. Later still, in an 
embryo 20-21 cm. long, the descent of the heart has changed once more 
these relationships leaving this area of the stem bronchus covered by the 
root of the pulmonary vein as it empties into the left auricle. As in the 
case of Lateral 1, this suppression represents a provision on the part of 
the tree to leave a space for the pulmonary veins. 
