628 



THE LUNGS. 



In Lepidosteus the air-bladder appears to arise, as in tlie Teleostei, 

 as an invagination of the dorsal wall of the oesophagus. 



In advanced embryos of Galeus, Mustelus and Acanthias, Miklucho- 

 Maclay detected a small diverticulum opening on the dorsal side of the 

 cesophagus, which he regards as a rudiment of a swimming bladder. This 

 interpretation must however be regarded as somewhat doubtful. 



The lungs. The lungs originate in a nearly identical way in all 

 the Vertebrate forms in which their development has been observed. 

 They are essentially buds or processes of the ventral wall of the 

 primitive oesophagus. 



At a point immediately behind the region of the visceral clefts 

 the cavity of the alimentary canal becomes compressed laterally, and 

 at the same time constricted in the middle, so that its transverse 

 section (fig. 418 i) is somewhat hourglass-shaped, and shews an upper 

 or dorsal chamber c?, joining on to a lower or ventral chamber Z by a 

 short narrow neck. 



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 communicate, but 

 the two lobes, partly by their 

 own growth, and partly by a pro- 

 cess of constriction, 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 oesophagus, carry- 

 ing with it the two lobes above 

 mentioned, becomes gradually trans- 

 formed into an independent tube, 

 opening in front by a narrow slit- 

 like aperture into the oesophagus. 

 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 hypoblastic 

 walls of the alimentary tract, the 

 splanclinic mesoblast surrounding 

 these structures becomes very much 

 thickened ; but otherwise bears no 

 marks of the internal changes 

 wdiich are going on, so that the 



7 



Fig. 418. Four diagrams illustra- 

 ting THE FORMATION OF THE LuNGK. 



(After Gotte.) 



a. mesoblast ; h. hypoblast ; d. cavity 

 of digestive canal; I. cavity of the pul- 

 monary diverticulum. 



In (1) 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 and 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 corre- 

 sponding to the internal changes whicli 

 have been going on. 



