FOEMATION OF THE LUXGS. 733 



owes its origin to both hyiioblastic and mesoljlastic elements. The 

 substance of the mesoblast, thickening round the primary diverticula, 

 becomes penetrated by secondary diverticula formed from the hypoblast 

 processes ; these are succeeded by tertiary branches which develop the 

 bronchia, and ultimately have the air-cells formed as their terminations. 

 The formative process consists essentially in the budding of hypoblastic 

 into mesoblastic substance ; the hypoblast furnishing the lining epi- 

 thelium of the tubes, and the mesoblast the other tissues, such as 

 muscular fibres, cartilage, blood-vessels, elastic tissue, c^-c. 



In the formation of the trachea and bronchi the wall of the primitive 

 oesophagus is projected downwards (or forwards), and by the gradual 

 folding in of the sides a second median tube is separated from the 

 primitive alimentary canal. This new tube grows out at its hinder 

 end so as to bulge at the two sides, and thus the commencement of a 

 right and left bronchus is formed. The subsequent division of the 

 diverticular hollow goes on by budding of the hypoblast from within 

 into the masses of pulmonary blastema. The division into larger lobes 

 externally, three in the right and two in the left lung, may be seen at 

 a very early period in the human foetus. As the bronchial subdivision 

 extends within the lungs, a tubercular or coarsely granular appearance 

 is seen over the outer surface, as observed by Kiilliker in the human 

 foetus in the latter half of the second montli. This is produced by the 

 primitive air-cells placed at the extremities of ramified tubes, which 

 occupy the whole of the interior of the organ : the ramification of the 

 bronchial twigs and multiplication of air-cells goes on increasing, and 

 this to such an extent that the air-cells in the fifth month are only 

 half the size of those which are found in the fourth month. 



Fig. 582. — Sketch iLLUSTRATiNa the Deve- Fig. 5S2. 



LOPMENT Oi THE RESPIRATORY ORGANS 



(from Fathke). A. B O 



A, oesophagus of a cliick, on the fourth 

 day of incubation, with the rudimentary 

 lung of the left side, seen laterally ; 1, the 

 front, and 2, the hack of the oesophagus ; 



3, rudimentary lung protruding from that 

 tube ; 4, stomach. B, the same seen in 

 front, so as to show both lungs. C, tongue 

 and resjjiratory organs of embryo of the 

 horse; 1, tongue; 2, larjmx ; 3, trachea j 



4, lungs seen from behind. 



In birds the principal air-sacs, three in number, are formed in direct 

 connection with the lung in the course of its early development, and 

 the riidiments of these sacs may be seen at an early period, as bulging 

 constituent parts of the rudimentary lungs. 



Pleurae. — Each lung receives a covering externally from the lining' 

 membrane of the common pleuro-peritoneal cavity of its own side. 

 This is at first only on the outer side; but, as the lungs enlarge, a fissure 

 separates their solid substance from the outer wall of the oesophagus, 

 and the pleura is carried round the lung-mass so as to encircle the 

 gradually narrowing root of each lung. The two pleurse remain sepa- 

 rated by the mediastinum and heart. 



Pulmonary Vessels.— The blood-vessels of the lungs which arise 

 in the mesoblastic tissue seem to be of comparatively late formation. 



