26 The Development of the Lungs 
organogenesis). In the upper part of the gut just below the gill 
pouches, the mesoderm, covered by ccelomic epithelium forms slight 
asymmetrical projections into the ccelom (Fig. 1 ¢), while at the level of 
pulmonary swelling, the anlage of the mesodermic portion of the lung 
wings (Fig. 2) takes the form of two irregular lateral projections into 
the coelomic cavity. The one on the right is much larger than that on 
the left (Fig. 2), so much so that at this stage the latter is only faintly 
shown. This results in a marked asymmetry of the primitive lung wings 
themselves. The mesoderm in the two wings is characterized by the rich- 
ness of its cellular content, as the portion behind the intestine already 
shows a differentiation preceding the stages of chondrification of the prim- 
itive vertebre. The mesoblastic anlage of the lungs arises from the 
general mesoderm of the head gut. Just below the pulmonary anlage 
TExT FKig. 3. 
Text Fig. 3. Section of embryo pig 3.5 mm. long, through Ductus hepaticus. 
C=Celom. DH =—Ductus hepaticus. 
Text Fig. 4. Section of embryo pig 4 mm. long at the beginning of the 
Mesocardium posterior. C—=Celom. VM—=Mesocardium posterior. SV = 
Sinus venosus. O=(£sophageal portion of head gut. PG Respiratory 
portion of the head gut. 
on the left side are evidences of the Recessus pleuroperitonalis which, 
as described by Stoss, 92, may at this stage be followed through a few 
sections. 
In a slightly later stage, 4 mm., for example, the embryo shows the 
next step in the development of the respiratory apparatus. The head 
gut is more symmetrical with reference to the median longitudinal plane 
(Figs. 4, 5, 6). In the upper portion below the gill pouches, a longi- 
tudinal fissure appears on either side dividing it now into well-marked 
dorsal and ventral segments giving the gut in the respiratory level, more 
or less of an hour-glass appearance in transsections. These fissures mark 
