v.] 



THE LUNGS. 



129 



section and of a larger bore than the oesophagus. Up to the 

 end of the day it is still completely closed at the hinder 

 extremity, which however is somewhat swollen to form the 

 cloaca. 



18. The lungs are in origin essentially buds or processes 

 from the primitive oesophagus. 



If the alimentary canal of the chick at the end of the 

 third day be dissected out and laid open, there will be seen 

 on either side of the hinder end of the oesophagus a short 

 pouch or diverticulum enveloped in a mass of mesoblast 

 (Fig. 42, Ig. ; here however they are somewhat advanced, 

 the specimen belonging to the fourth day). These pouches 

 are the early rudiments of the lungs. Their mode of origin 

 is as follows. 



Fig. 43. 



FOUR DIAGRAMS ILLUSTRATING THE FORMATION OF THE LUNGS. 



(Copied from Gotte.) 



a. mesoblast. h. hypoblast, d. cavity of digestive canal. I. cavity of the 

 pulmonary diverticulum. 



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

 and lower canal ; the former the true alimentary canal, the latter the pulmonary 

 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 which have been going on. 



E. 9 



