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The Development of the Lungs 
At 10 mm. the two simple lungs are quite asymmetrical (Fig. 15). 
Increasing in size with the growth of the bronchi, they also follow their 
asymmetrical development. The fain swellings observed in the preceding 
stage have become so exaggerated that we have on the surfac of the lung 
marked rounded elevations indicating the presence of Lateral 1 (Fig. 15, 
L. 1) on the right side, and Lateral 2 on both sides (Fig. 15, L. 2). 
These projections are limited by shallow groves. From above down- 
wards, the trachea and hence the mesoderm extends ventralwards until 
the point of bifurcation is reached, when, following the course of the 
stem bronchi, it passes dorsalwards on either side of the cesophagus. 
At 12.5 mm. (Fig. 16) these characteristics are exaggerated. On 
the right side, high up, we have the projections over the bronchi, which 
have been found before this stage. ‘They have increased in size with the 
7 l1 
1 
Le Le Le 
Le | 
fi Vo L3 13 (2 L3 
3 4 
L4 te 14 
14 
A. 
1B}. 
Adios 1a alir/ 
TExt Fic. 17. Outline drawing of the lungs of an embryo pig 13.5 mm. 
long. A. Ventral view. B. Dorsal view. The letters represent the mesodermic 
swellings over the bronchi designated by the abbreviations. Designations 
the same as in Fig. 16, except that L.3 and all swellings below that order 
unite in the pig to form the Lobus inferior. 
growth of their respective elements; also there is now a well-marked pro- 
jection over the newly-formed V. 2 (Fig. 16, V. 2) and a less apparent 
swelling, the bud representing Lateral 3 on each side (Fig. 16, L. 3). 
The furrows have deepened, and the lower part of the wings below Ven- 
tral 2 now embraced by the Wolffian body and chest wall dorsally, the 
heart, liver, and diaphragm ventrally, and the mesoderm of the ceso- 
phagus medially, have already in cross-sections an irregular prismatic 
form. At this stage we may say, the anlagen of the lobes are com- 
plete. From each of these main projections, a lobe is produced and the 
shallow grooves deepen with the further growth of the lungs to form 
the interlobar fissures. That is to say, on the right side the swellings 
over Lateral 1, Lateral 2, Ventral 2, and the stem produce respectively 
