870 DEVELOPMENT [CH. LIX. 



the walls of the fore-gut, in the form of two conical hollow branches 

 (figs. 668, 669). The inner portion of the cones consists of a number 

 of solid cylindrical masses of cells, derived from the hypoblast, which 

 become gradually hollowed by the formation of the hepatic ducts, 

 and among which blood-vessels are rapidly developed. The secreting 

 cells of the organ and the lining epithelium of the ducts are derived 

 from the hypoblast; the connective tissue, and vessels from the 

 mesoblast. 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 a diverti- 



FIG. 669. Rudiments of the liver on the intestine of a chick at the fifth day of incubation. 1,'Heart ; 

 2, intestine; 3, diverticulum of the intestine in which the liver (4) is developed; 5, part of the 

 mucous layer of the germinal membrane. (Miiller.) 



culum from the floor of the fore-gut, opposite the first clefts, and by 

 two diverticula from the fourth visceral clefts. 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 way from the third visceral clefts, and its hypoblastic cells 

 form the corpuscles of Hassall; the lymphoid tissue by which they 

 are invaded and ultimately surrounded is mesoblastic. 



Development of the Respiratory Apparatus. 



The Lungs first appear as two small diverticula from a groove 

 in the ventral wall of the fore-gut (figs. 668, 670). 



The groove is gradually separated off to form the trachea and 

 larynx, and the diverticula becomes the bronchi, whilst the dorsal 

 part of the fore-gut in this region forms the oesophagus. These 

 primary bronchial diverticula of the hypoblast of the alimentary 

 canal send off secondary branches into the surrounding mesoblast, 

 and these again give off tertiary branches, forming the air-cells. Thus 



