764 



THE LUNGS. 



The hinder end of the lower tube enlarges (fig. 418 2), and 

 then becomes partially divided into two lobes (fig. 418 3). All 

 these parts at first freely com- 

 municate, but the two lobes, 

 partly by their own growth, 

 and partly by a process of con- 

 striction, soon become isolated 

 posteriorly; while in front they 

 open into the lower chamber 

 of the oesophagus (fig. 422). 



By a continuation forwards 

 of the process of constriction 

 the lower chamber of the ceso- 

 phagus, carrying with it the 

 two lobes above mentioned, 

 becomes gradually transformed 

 into an independent tube, 

 opening in front by a narrow 

 slit-like aperture into the oeso- 

 phagus. The single tube in 

 front is the rudiment of the 

 trachea and larynx, while the 

 two diverticula behind become 

 (fig. 419, Ig) the bronchial tubes 

 and lungs. 



While the above changes 

 are taking place in the hypo- 

 blastic walls of the alimentary 

 tract, the splanchnic mesoblast 

 surrounding these structures 



FlG. 418. FOUR DIAGRAMS ILLUSTRA- 

 TING THE FORMATION OF THE LUNGS. 



(After Gottc.) 



a, mesoblast; b. hypoblast; d. cavity 

 uf digestive canal ; /. cavity of the pul- 

 monary diverticulum. 



In (i) the digestive canal has com- 

 menced to be constricted into an upper 

 and lower canal ; the former the true 

 alimentary canal, the latter the pulmo- 

 nary tube; the two tubes communicate 

 with each other in the centre. 



In (2) the lower (pulmonary) tube has 

 become expanded. 



In (3) the expanded portion of the 

 tube has become constricted into two 

 tubes, still communicating with each other 

 anil with the digestive canal. 



In (4) these are completely separated 

 from each other and from the digestive 

 canal, and the mesoblast has also begun 

 to exhibit externally changes correspond- 

 ing to the internal changes which have 

 been going on. 



becomes very much thickened ; but otherwise bears no marks of 

 the internal changes which are going on, so that the above 

 formation of the lungs and trachea cannot be seen from the 

 surface. As the paired diverticula of the lungs grow backwards, 

 the mesoblast around them takes however the form of two lobes, 

 into which they gradually bore their way. 



There do not seem to be any essential differences in the mode of 

 formation of the above structures in the types so far observed, viz. Amphibia, 

 Aves and Mammalia. Writers differ as to whether the lungs first arise as 



