8$4 DEVELOPMENT. [CH. LVIII. 



developed. The secreting cells of the organ and the lining 

 epithelium of the ducts are derived from the hypoblast, the con- 

 nective-tissue and vessels from the rnesoblast. The gall-bladder 

 is developed as a diverticulum from the hepatic duct. 



The spleen and lymphatic glands are developed from the meso- 

 blast : the thyroid originates from the hypoblast ; it grows as 

 diverticula from the fore-gut, opposite the second and also opposite 

 the fourth visceral arches. The hypoblastic cells form the lining 

 epithelium of the vesicles ; the stroma of the gland is formed by 

 the surrounding mesoblast. The thymus is formed in a similar 



Fig. 673. Rudiments of the liver on the intestine of a chick at the fifth day of incubation, 

 i, heart ; 2, intestine ; 3, diverticulum of the intestine in which the fiver (4) is deve- 

 loped ; 5, part of the mucous layer of the germinal membrane. (Miiller.) 



way opposite the third and fourth visceral arches. These hypo- 

 blastic cells form the nests called the corpuscles of Hassall ; the 

 lymphoid tissue by which they are invaded and ultimately sur- 

 rounded is mesoblastic. 



Development of the Respiratory Apparatus. 



The Lungs, at their first development, appear as small tuber- 

 cles or diverticula from the ventral surface of the oesophagus 

 (figs. 672, 674). 



The two diverticula at first open directly into the oesophagus, 

 but as they grow, a separate tube (the future trachea) is formed 

 at their point of fusion, opening into the oesophagus on its anterior 

 surface. These primary diverticula of the hypoblast of the ali- 

 mentary canal send off" secondary branches into the surrounding 

 mesoblast, and these again give off tertiary branches, forming the 

 air cells. Thus we have the lungs formed : the epithelium lining 

 the air-cells, bronchi, and trachea is derived from the hypoblast, 

 and all the rest of the lung-tissue, nerves, lymphatics, and 



