178 THE THIED DAT. [CHAP. 



mesoblast surrounding these structures 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. 



The further development of the lungs is, at first, 

 essentially similar to that of a racemose gland. From 

 each primitive diverticulum numerous branches are 

 given off. These branches, which are mainly confined 

 to the dorsal and lateral parts, penetrate into the sur- 

 rounding mesoblast and continue to give rise to second- 

 ary and tertiary branches. At right angles to the 

 finest of these the arborescent branches so charac- 

 teristic of the avian lung are given off. In the meso- 

 blast around them numerous capillaries make their 

 appearance. 



The air sacs, which form such important adjuncts 

 of the avian lungs, are the dilated extremities of 

 the primary pulmonary diverticula and of their main 

 branches. 



The whole pulmonary structure is therefore the 

 result of the growth by budding of a system of branched 

 hypoblastic tubes in the midst of a mass of mesoblastic 

 tissue, the hypoblastic elements giving rise to the epi- 

 thelium of the tubes and the mesoblast providing the 

 elastic, muscular, cartilaginous, connective and other 

 tissues of the tracheal and bronchial walls. 



The liver is the first formed chylopoietic appendage 

 of the digestive canal, and arises between the 55th and 



