50 The Development of the Lungs 
bud-like projections, one of which represents the median branch (PI. ILI, 
Fig. 20, D. 2, m), usually the first branch of the dorsal series, which is 
already indicated in the preceding construction. Lateral 3 (Pl. II, Fig. 
19; Pl. III, Fig. 20, L. 3) passes lateralwards and slightly ventralwards. 
It has a well-marked dorsal (Pl. III, Fig. 20, L. 3, d) and somewhat 
further out a ventrosuperior branch (Pl. II, Fig. 19; Pl. Il, Fig. 20, 
L. 3, vs), both of which are represented in the adult lung. The continu- 
ation of the bronchus ends in a bud, which is already undergoing further 
division. At a point just above the fourth lateral, Ventral 3 (Pl. II, 
Fig. 19, V. 3) arises, and ends in a slight terminal swelling. Dorsal 3 
(Pl. III, Fig. 20, D. 3) is considerably smaller than D. 2, and also 
smaller than the corresponding branch on the opposite side, but is 
already divided into two buds, one of which represents the median branch 
of this bronchus, while the other forms the stem. Such variations 
in size as are shown in this instance, however, occur very frequently. 
Lateral 4 is somewhat shorter than L. 3, and has a well-marked ventral 
and a less marked dorsosuperior branch. The fourth ventral bron- 
chus (Pl. II, Fig. 19, V. 4) is slightly smaller than the third and 
arises from the corresponding position in this interspace, while D. 4 
(Pl. Ill, Fig. 20, D. 4) is considerably longer than the third, and 
ends in a bud which is not yet divided. Lateral 5 (Pl. Il, Fig. 19; 
Pl. Ill, Fig. 20, L. 5) terminates in an undivided bud, and V. 5 (PI. 
II, Fig. 19, V.5) consists simply of a slight bulging of the epithelial 
wall of the axial bronchus. Similarly the fifth dorsal (Pl. III, Fig. 20, 
D.5) is merely suggested by a faint projection from the epithelial tube. 
Lateral 6 (Pl. II, Fig. 19; Pl. III, Fig. 20, L. 6) is the smallest of the 
lateral series and ends in a slight swelling, while the axial bronchus con- 
tinues downwards, terminating in an end bud. At this point the division 
of the stem is practically dichotomous. This specimen has no medial 
bronchi and is especially characterized by the lack of variations, for all 
of the bronchi, excepting the medial group, are present in almost 
schematic order. The entire absence of the medial group, however, must 
be regarded as exceptional for most trees, either on one side or both, have 
medial branches in some of the interspaces below the level of Lateral 4. 
While we have seen in the reconstructed series, examples of variations 
caused by the suppression of either a dorsal or ventral bronchus, another 
type occurs, not represented here, of which I have several specimens in 
my corrosions of the embryonic lung, namely, a reduplication of either 
the dorsal, ventral, or the medial bronchi in any one interspace. This 
may or may not be accompanied by a simultaneous suppression of one 
