Joseph Marshall Flint 31 
phagus and trachea are separated by a mass of mesoderm filling the 
intervening spaces. The simple bronchial system has increased in length 
and caliber, but the relationships are practically the same, save for the 
appearance of the rounded terminal buds on the end of the stem bronchi 
(PL. I, Figs 5, 6ds). While, in this embryo, the two bronchi still lie 
ventralwards to the head gut, they now begin at the ends to bend more 
dorsalwards than in the preceding stage, the right a trifle more than the 
left. The Mesocardium posterior is still lower than in the preceding 
stage, its upper level now beginning only a short distance above the 
origin of the left bronchus. The mesodermic syncytium is unchanged. 
The lung wings are fairly symmetrical as they project on either side 
into the ccelomic cavity. The difference, however, between the right and 
left lung bronchi still suffice to give the two lungs a slight asymmetry. 
The Recessus pleuroperitonealis is marked on the right side and ex- 
Thx Bre, 10: 
Text Fig. 10. Section through the primitive lung sacs of an embryo 7.5 
mm. long. C=Celom. BD=Right stem bronchus. BS—=Left stem 
bronchus. DM Dorsal mesentery. VM —=Mesocardium posterior. 
tends some distance above the lower end of the right bronchus, while the 
left recessus is almost obliterated. 
The next stage in the development shown in an embryo 7.5 mm. long 
consists in the complete production of the primitive lung sacs through 
the dilatation of the buds on the end of the right and left bronchi (PI. I, 
Figs. 7, 8sd). The size of the branches of the primitive tree have 
increased markedly, the two dilated lung sacs while still lying ventral- 
wards of the cesophagus now bend sharply backwards forming a horse- 
shoe-like curve around it (Fig. 10). The left still preserves its position 
at right angles to the trachea with a slight growth caudalwards at the 
bottom of the sac. On the right side, the general direction of the 
bronchus is lateralwards, dorsalwards, and caudalwards. The form of 
