116 The Development of the Lungs 
in others the left only may atrophy, while in still others both may undergo 
the atrophic changes leaving the tree consisting only of a symmetrical 
hyparterial system. This observation of d’Hardiviller has only received 
a single supporting observation in the whole literature and that is by 
Bremer in the opossum lung. Bremer finds in embryos of 12.5 mm. 
what he calls an eparterial bronchus on the left side. In his specimens, 
14 cm. long, this is absent and, therefore, he presumes the bronchus has 
degenerated between the two stages he has been able to observe. Narath, 
in the possession of two adult rabbit lungs with left eparterial bronchi 
as variations, is inclined to believe d’Hardiviller is dealing with an 
abnormality, and, furthermore, in view of the unique nature of the 
observation, adds that absolutely indisputable histological preparations 
must be produced to show the degeneration of a bronchial bud which has 
once been formed. This criticism of Narath would, in part, apply to 
Bremer’s observation. The production of the bronchial tree in Echidna, 
according to Narath, follows the same principles which we observe in 
other mammals and the lung of the adult is not differentiated from that 
of placentalia. Moreover, the vessels and their relationships undergo no 
further changes while the young are in the pouch either in respect to the 
artery or the veins. It is thus hardly possible in these observations of 
d’Hardiviller and Bremer that we are dealing with a true regressive pro- 
cess. In fact, it is more probable that in both cases we are either dealing 
with a variation or a dorsal bronchus which is placed higher up than 
usual upon the stem bronchus. This assumption is made quite probable 
by Bremer’s statement that his left eparterial bronchus did not supply 
the apex of the lung. 
This bronchus is undoubtedly one of the lateral series as Zumstein 
and Nicholas and Dimitrova hold. It, like the remainder of the lateral 
series, originates from the lateral wall of the trachea or the stem. The 
fact that it is usually unpaired and has a different topography to the 
pulmonary artery does not separate it from this group. It is true, the 
bronchus originates a little more dorsalwards than the remainder of the 
series, but this is due partly to the different space relationships in the 
upper part of the thorax and partly, to the ventral torsion of the lower 
lateral bronchi, which exaggerates the slight difference that occurs be- 
tween Lateral 1 and the remainder of the series in the embryo. 
Inasmuch as a bronchus corresponding to Lateral 1 has never been 
described in Reptilia or Amphibia, it must be regarded as peculiar to 
mammals. The great rarity in the occurrence of paired first lateral 
bronchi suggests that no more morphological significance can be laid on 
