40 The Development of the Lungs 
length and passes ventralwards, medianwards, and caudalwards. At its 
terminus there is a definite bud. The corresponding portion of the stem 
bronchus on the left side, however, remains nude. In seeking for an 
explanation of the cause for the extreme development of Ventral 2 on the 
right side and its usual absence on the left, I have been impressed with 
the extreme adaptability of the lung to its environment and the way in 
which the bronchi follow mechanical principles in growing along the 
lines of least resistance. We realize, of course, the fact that the lungs 
are relatively late accessions to the animal economy, that they also, ex- 
cepting possibly in marsupials, are functionless until the period of 
birth. It is natural, therefore, to find them secondary to and influenced 
by such organs as the heart and liver, as well as the chest wall by which 
they are surrounded. These, moreover, have chronologically the de- 
velopmental precedence and are of definite functional use during the 
embryonic life of the organism. For the suppression of left V. 2 there is 
an explanation as we shall see in the chapter on the development of the 
pulmonary vessels and we may look upon the hyperdevelopment of right 
V. 2 as an effort to fill up the space which exists especially in quadrupeds 
between the heart and diaphragm in the region of the median plane. 
Dorsal 2 (Pl. I, Fig. 16, D. 2), situated between L. 2 and L. 3, shows a 
slight growth over the preceding stages, but still persists simply as a 
slight projection from the axial bronchus. The third lateral (PI. IT, 
Figs. 15, 16, L. 3) has increased in size over the corresponding branch 
in a younger embryo, and now possesses a more distinct terminal bud. 
There is, however, no indication of division as yet. On the ventral side 
of the axial bronchus, just beneath L. 3, there appears a slight swelling, 
indicating the origin of the third ventral bronchus (Pl. II, Fig. 15, 
V. 3). Directly behind it, on the dorsal surface of the stem, is a pro- 
tuberance showing the point of origin of Dorsal 3 (Pl. II, Fig. 16, D. 3). 
Below these two branches, there is on the lateral side of the axial 
bronchus a bud indicating the point of origin of Lateral 4 (Pl. II, Figs. 
15, 16, L. 4), while the axial bronchus continues downwards and ends in a 
terminal swelling on which some signs of the origin of L. 5 (PI. II, 
Figs. 15, 16, L. 5) are already shown. 
At this level, an evagination (Pl. II, Figs. 15, 16 MS) appears on the 
inner side of the end bud pointing medialwards and slightly dorsalwards 
just opposite the bud of Lateral 5. This is the first one of the medial 
series to appear on the reconstructions. ‘They are, however, extremely 
variable both in their constancy and origin. In some trees they are en- 
tirely absent, in others they may occur with great regularity, but never 
in my specimens, which included sections and corrosions of over one hun- 
