PULMONARY EVOLUTION IN MAMMALIA 101 
in the relatively small number of lving forms with bilateral 
eparterial bronchial trees. From it Aeby derived all the re- 
maining types by reduction and final suppression of the epar- 
terial component on the left side in the vast majority of the 
living mammalia, or on both sides in the only instance of a bi- 
lateral hyparterial tree then known to exist, Hystrix cristata. 
Aeby hence regarded the mammalian lung as having undergone, 
in its modern prevalent type, a process of devolution or regres- 
sion, to a greater or lesser degree, and his hypothesis can hence 
be briefly defined as the ‘Reduction Theory.’ 
At the time of Aeby’s publication the embryology of the 
mammalia and of the vertebrates generally was imperfectly 
known, and he confidently looked forward to the support of his 
hypothesis through future ontogenetic investigations. He was 
disappointed in the first of these when His (24) in 1887, seven 
years after the appearance of Aeby’s work, published his ac- 
count of the development of the human lung. But Aeby’s view 
received apparently absolute confirmation in the embryological 
evidence adduced by d’Hardiviller (13, 14, 18) in 1897. ‘This ob- 
server, who published, between December, 1896, and December, 
1897, no less than eight papers (11, 12, 13, 14, 15, 16, 17, 18) 
on the development of the mammalian bronchial tree and the 
homologies of its derivatives in sheep, rabbit and man, describes 
his findings in rabbit embryos of 13 days and 6 hours as follows: 
The right lung shows three main bronchial buds: 
1. The eparterial bronchial bud for the upper lobe, carrying a 
secondary lateral derivative. 
2. The bud for the middle lobe. 
3. The continuation of the primitive stembronchus into the 
lower lobe, with a lateral and medial secondary bud, the latter 
the anlage of the infracardiac bronchus. 
On the left side he finds: 
1. The bud for the upper lobe. 
2. The bud for the lower lobe, formed by the continuation of 
the main stembronchus, with a lateral derivative. 
3. A ‘hollow epithelial vesicle’ arising from the left stem- 
bronchus, cranial to the bud for the upper lobe and directed 
cranio-dorsad. 
