30 The Development of the Lungs 
right and left bronchi. Of the two, the left (Pl. I, Fig. 3s) passes 
lateralwards almost at right angles to the axis of the groove, while the 
right (Pl. I, Fig. 3d) extends caudalwards and lateralwards, giving 
a sharp asymmetry to the fork which they form with the trachea (PI. I, 
Fig. 37’). From the slight crest in the midline which is not seen in 
the ventral view, both bronchi bend slightly dorsalwards. At the ends, 
there is a slight increase in the caliber of the bronchi, but end buds are 
not yet formed upon them. Underneath the point where the two unite, 
the sulcus from which the separation begins is already present, but it 
does not extend quite as far cranialwards as in the preceding stage. 
Viewed in profile, the whole anlage now extends somewhat ventralwards 
from the head gut, an extremely important relationship as we shall see 
in the chapter on the relation of the blood-vessels to the bronchial tree 
(cf. Schema A). The head gut below the origin of the two bronchi bends 
slightly ventralwards and to the left. In this region, which may be 
considered the anlage of the stomach, a noticeable dilatation of the gut 
is taking place (PI. I, Figs. 3, 4). 
In this stage the character of the mesoderm has not changed, the 
Mesocardium posterior begins at a lower level owing to the descent ox 
the heart, while the dorsal mesentery is now well marked above the 
level of the lower extremity of the right bronchus. The two lung wings 
are more symmetrical and project further into the ccelom than in the 
preceding stage. Nevertheless, they are still asymmetrical in so far as 
the projection forming the left lung is higher than that of the right. 
Both on the right and left sides, the Recessus pleuroperitonealis is well 
marked. In another embryo of the same measurement, but evidently 
somewhat better developed, the process of separation of the bronchi from 
the cesophagus is well started. The sulcus between the trachea and the 
cesophagus extends just above the level of the origin of the two bronchi. 
This is filled with mesoderm of a nature similar to that about the head 
gut. The mechanical factors involved in the process are difficult to 
make out, but it begins by an approximation of the epithelium along the 
line of the two lateral fissures and then proceeds upwards from the 
suleus formed behind the primitive bronchi which is filled with mesoderm. 
At this stage the following formula of the derivatives of the pulmonary 
anlage may be made: 
TRACHEA. 
Right bronchus. Left bronchus. 
At 6 mm. (PI. I, Figs. 5, 6) the process of separation is practically 
complete, the trachea and cesophagus remaining in communication only 
at the upper end. At the point of origin of the two bronchi, the cesa- 
