164 EMBEYOLOGY OF THE LOWEB VEBTEBEATES oh. 



established a series of channels running in a dorsiventral direction 

 through the substance of the lung and communicating dorsally with 

 the ectobronchi and ventrally with the entobronchi. The channels 

 in question are termed parabronchi (Fig. 91, par). These are 

 embedded in an abundant matrix of mesenchyme which from about 

 the tenth day becomes divided up into more or less prismatic 

 masses each having in its axis an individual parabronchus — the 

 prisms being delimited from one another by the development of 

 intervening blood-vessels. The mesenchyme which constitutes the 

 inner portion of this sheath round each parabronchus becomes later 

 replaced by a layer of smooth muscle fibres. 



At about the same period as the fusion of the parabronchial tips 

 takes place, the wall of the parabronchus begins to grow out into 

 numerous little pockets arranged in radiating fashion. These extend 

 outwards, perforating the muscular sheath, and at a short distance 

 from the parabronchus divide into branches which in turn elongate 

 and become the air-capillaries of the fully developed lung. Judging 

 from adult structure it would appear that the tips of these fuse 

 with others to form the continuous air-capillaries so that the latter 

 would be formed much in the same way as the parabronchi but 

 it has not been possible, so far, to demonstrate this by actual 

 observation. 



The essential features of the development of the Bird's lung as 

 above outlined may be summed up in the statement that in this 

 type of lung the diverticula of the intrapulmonary bronchus, which 

 in other Vertebrates end blindly, become here joined together tip 

 to tip to form continuous tubular channels. To allow this arrange- 

 ment to function efficiently an apparatus is needed to force the air 

 through the system of respiratory tubes: such an apparatus is 

 provided by the air-sacs. 



Air-sacs. — The ventral part of the lung-rudiment is for a time 

 formed of a thick mass of mesenchymatous tissue which has heen 

 termed by Bertelli the primary diaphragm, from the fact that it 

 becomes continuous along its lateral margin with the side wall of 

 the splanchnocoele, so as to form a kind of floor separating off the 

 lung from the splanchnocoele which lies ventral to it. The air-sacs 

 arise as outgrowths of the bronchial cavities and are on each side 

 four in number : the first or most anterior giving rise to the cervical 

 sac, the second by bifurcation to interclavicular and anterior 

 thoracic sacs, the third to the posterior thoracic and the fourth to 

 the abdominal sac. The rudiments sprout out into the substance 

 of the primary diaphragm and become greatly distended within it, 

 bulging out ventrally amongst the viscera so that the ventral layer 

 of the diaphragm becomes stretched < out into a thin membranous 

 wall delimiting the cavity of the air-sac on its ventral side. The 

 dorsal part of the primary diaphragm, lying above the air-sacs 

 persists as the floor of the lung or secondary diaphragm (ornithic 

 diaphragm of Bertelli, pulmonary aponeurosis of Huxley). 



