Joseph Marshall Flint 53 
(2) D. (2) 8. 
(2) V (2) D 
(2) V 
D. 4. D. 4 
(2) M. M. 
(2) L (2) L 
V. 4. Vv. 4 
(2) M. (2) M 
Tes 5 L. 5 
(2) V. (2) D 
(2) DS. (2) V 
(2) I (2) DS. 
IDES D5 
(2) M. (2) M. 
V.5 Wa 15) 
M. 5 M. 5 
L. 6 L. 6 
(2) V 
(22D 
D6: D. 6 
V.6 V.6 
In a pig 5 em. long, the bronchial tree can be studied by celluloid cor- 
rosions (Pl. IV, Fig. 21), but perfect specimens of the air passages in 
these small embryos are extremely difficult to obtain. The main features 
of the tree remain practically the same as in the earlier stages, save 
that it has increased markedly in the complexity of its branching. 'l'he 
trachea with its main bronchi maintains the wish-bone appearance op- 
served in the reconstructions of younger embryos, but a marked difference 
is noted in the lateral bronchi, which now bend sharply ventralwards as 
the lung folds around the heart and liver, following the curve of the 
thoracic wall. The first lateral bronchus, while showing the chief charac- 
teristics observed in the younger stages, has a more complicated system 
of branches. It extends lateralwards and posterior, and divides into 1ts 
two main branches, the dorsoinferior and ventrosuperior. The former 
runs dorsalwards, ventralwards, and posterior, while the latter brancn 
passes anterior, ventralwards, and slightly medianwards. The maim 
branches of the dorsoinferior bronchus are, at this stage, seven in number, 
and extend dorsally, laterally, and medially. Their serial arrangement 
may be determined from the tabulation at the end of this section. 'I'here 
are five main branches of the ventrosuperior or apical division, which 
have chiefly a dorsosuperior and a lateroinferior course. 
Lateral 2 on the right side shows a marked increase in the complexity 
of its large dorsoinferior bronchus, which now shows six subdivisions. 
