VI.] 



THE LUNGS. 



177 



and larynx, while the two diverticula behind (Fig. 60, 

 Ig) become the bronchial tubes and lungs. 



While the above changes are taking place in the 

 hypoblastic walls of the alimentary tract, the splanchnic 



Fig. 62. 



a. 



Four Diagrams illustrating the Formation of the 

 Lungs. (After Gotte.) 



mesoblast ; b. hypoblast ; d. cavity of digestive canal ; I. 

 cavity of the pulmonary diverticulum. 



In (1) the digestive canal has commenced to be constricted 

 into a dorsal and ventral canal ; the former the true alimentary 

 canal, the latter the pulmonary tube ; the two tubes communi- 

 cate with each other in the centre. 



In (2) the ventral (pulmonary) tube has become expanded. 



In (3) the expanded portion of the tube has become con- 

 stricted 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 corresponding to the internal changes 

 which have been going on. 



F. (%B. 12 



