Ill 



LUNG 



163 



eel 



being the rudiment of a lung in the restricted sense and containing 

 a prolongation of the enteric cavity lined by tall columnar endoderm 

 cells. Outside the endoderm is a thick layer of mesenchyme and 

 this in turn is covered by columnar coelomic epithelium. 



The endoderm -lined cavity is destined to become the main 

 intrapulmonary bronchus— the mesobronchus. This remains un- 

 branched until the fifth day when its endoderm begins to bulge 

 out, near the point where it enters the lung, to form the first ento- 

 bronchus. During further development a series of three other 

 entobronchial outgrowths sprout out from the external surface of 

 the mesobronchus close behind the first outgrowth. The four ento- 

 bronchi so arising are closely contiguous and form a longitudinal 

 row (Fig. 92, E1-E4). 



A set of similar outgrowths make their appearance spaced out 

 along the mesial side of the mesobronchus posterior to the ento- 

 bronchi : these are the rudiments of the ectobronchi. A third set of 

 outgrowths on the lateral 

 side of the mesobronchus 

 are the rudiments of the 

 small secondary lateral 

 bronchi (Campana). Of 

 these sets of outgrowths 

 the first and second are 

 the most important and 

 they are arranged in a 

 slightly spiral row along 

 the wall of the meso- 

 bronchus. 



The mesobronchus, as 

 it grows in length, 

 assumes a somewhat 

 'Xj- shaped curvature, by 

 which the group of ecto- 

 bronchi are carried towards the dorsal face of the lung while the 

 entobronchi are nearer the ventral surface (cf. Fig. 91). Both ecto- 

 bronchi and entobronchi grow rapidly parallel with and close to 

 the surface of the lung -rudiment. They soon produce secondary 

 branches as projections of their walls and these secondary branches 

 increase greatly in length traversing the substance of the lung at 

 first close to its median surface and, later, deep down in its sub- 

 stance as well— the entobronchial branches growing in a dorsal and 

 the ectobranchial in a ventral direction. 



The two sets of branches as their tips approach one another are 

 seen to alternate in position (Fig. 91). When they have approached 

 closely each branch bifurcates and its two tips become closely 

 apposed to the two tips belonging to the other serj.es which lie 

 closest to them. About the thirteenth day these apposed tips become 

 completely fused and their cavities continuous so that there is now 



Fig. 91. — Diagram illustrating the arrangement of the 

 main air-passages in the lung of the Fowl as seen 

 from the mesial plane. (After Juillet, 1912.) 



br, bronchus ; ect, ectobronchi ; ent, entobronchi ; 

 tries, mesobronchus-; par t parabronchi. 



