10 The Development of the Lungs 
rent belief, as he himself points out, the division of the bronchi was 
dichotomous. Little of the origin, the relations, and mode of division 
of the bronchi was known and even less of the significance of the lobes 
either to each other or to the species in which they were found. Aeby, 
80, graphically describes the darkness which surrounded our knowledge 
of the lung and blames the widely-accepted dogma of dichotomy for the 
condition. It is noteworthy, however, how the few objective investigators 
whose publications immediately preceded Aeby’s also held his conception 
of the growth process of the tree. Among the first of these was Kiittner, 
76, who followed certain stages of the growth of the bronchi in the older 
stages of cow embryos and described the method of their proliferation as 
undivided from the end, that is to say, monopodial. From the stems 
of the bronchi, he says, lateral buds appear having their axes directed 
at right angles to the mother bronchus. By the subsequent rapid growth 
of these branches the monopodial character of the division is lost and an 
apparent dichotomy ensues. A year later Cadiat, 77, in sheep embryos 
measuring 12-15 mm. and upwards finds the trachea and main bronchi 
already well formed and describes the growth process as occurring not 
from the dilated ampullae at the end of the bronchi but rather from 
lateral outgrowths from their walls. In a slightly different way Stieda, 
78, who also used sheep embryos supplemented by rabbits, came to prac- 
tically the same conclusion. 
In the year preceding Aeby’s publication, Kolliker, 79, describes the ap- 
pearance of secondary branches upon the primitive lung sacs in rabbits 
on the 12th day, when the stem bronchus of each lung has three pro- 
jections. From this period the subdivisions become so numerous that it 
is difficult to follow them step by step, but, in general, the first branches 
pass dorsalwards and lateralwards. This branching, according to K6l- 
liker, occurs from hollow buds or projections from the epithelial tube 
which multiply rapidly until each lung consists of a small tree of hollow 
canals with swollen terminal buds. 
From these citations it is of course obvious that the idea of monopody 
was not new at the time Aeby wrote, and so the ignorance of the times 
concerning the architecture of the pulmonary tree was not, as Aeby sup- 
posed, so much due to the dogma of dichotomy as to the lack of a thorough 
piece of objective research such as he himself attempted to supply. 
And while many of his conclusions may find no place in our final con- 
ceptions concerning the structure of the lung, still they must always re- 
ceive the credit of having furnished us with a working hypothesis by 
the aid of which the problem might be attacked by objective methods. 
His suggestive appeal to embryologists, of which His, 87, speaks later, 
