Development of the Third Day 209 



trachea and oesophagus are continuous, and at 

 this point will be the &//!& 



As the two lung rudiments grow backwards, 

 the mesoblast gradually collects around them 

 as two lobes (Fig. 69, BP), and it is from the 

 mesoblast of these lobes that the elastic, 

 muscular, cartilaginous and other tissues of 

 the lungs and bronchial tubes are formed. The 

 epithelial lining of the lungs, down to the 

 smallest air-cells, is formed by the continuous 

 branching of the two original entoblastic out- 

 growths from the oesophagus ; so that while 

 the chief thickness of the walls of the lungs, as 

 well as their blood vessels, is formed of meso- 

 blast, the entire epithelial lining is of ento- 

 blastic origin. 



" The air sacs, which are structures very characteris- 

 tic of birds, appear about the eighth day as thin-walled 

 saccular diverticula from the hinder edges of the lungs." ' 



The liver. The origin of the liver, as a 

 median outgrowth from the floor of the diges- 

 tive tract, was briefly described in the case of 

 the frog : its development in the chick will be 

 described more in detail. 



" The liver arises, about the middle of the third day, 

 1 Marshall. 



