28 The Development of the Lungs 
While the two wings still show the influence of the asymmetry of the 
bronchial projections, they are somewhat more regular than in the 
preceding stage. The anlage of the right wing is larger than the left 
and the Mesocardium posterior is also pushed slightly to the right. The 
character of the mesoderm remains about the same as in the last stage, 
that is to say, rich in cells with scarcely any differentiation of the exo- 
plasm into primitive connective-tissue fibrils. Below the lung anlage, 
the Recessus pleuro-peritonealis is patent on the right side. 
In a still later stage, 4.5 mm., the conditions remain practically as 
in an embryo of 4mm. ‘The most apparent differences lie in the further 
development of the two main bronchi. That on the left (Fig. 8 BS) 
grows practically at right angles to the axis of the pulmonary groove, 
while the right bronchus is directed laterally and caudally (Fig. 9 BD) 
Thx Wie. 8. 
Text Fic. 7. Section of an embryo pig 4.5 mm. long at the beginning 
of the Mesocardium posterior. C—=Celom. PG=Respiratory portion of the 
head gut. O=Digestive portion of the head gut. SV—Sinus venosus. 
Text Fic. 8. Section of an embryo 4.5 mm. long, through the anlage of the 
stem bronchi. C=Celom. PA=Pulmonary anlage. BS=Left stem 
bronchus. O = (Msophagus. 
and extends through a number of sections after the other has disap- 
peared.” From the anlage at the point of origin of the bronchi, there is 
a crest-like projection of the epithelial tube in the midline which is 
exaggerated by the slight dorsal flexure of the two main bronchi. This is 
scarcely seen in cross-sections, but can be made out easily in embryos 
cut longitudinally. At this stage, we also note the beginning of the pro- 
cess of separation of the respiratory from the digestive tract in a sulcus 
(Fig. 8) formed below the pulmonary anlage just behind it and in front 
of the ventral part of the cesophagus which is continuous above with the 
lateral fissures. In this particular embryo, the process seems a little 
