706 HANDBOOK OF PHYSIOLOGY. 



the formation of the hepatic ducts, and among which blood-vessels are 

 rapidly developed. The gland-cells of the organs are derived from the 

 hypoblast, the connective tissue and vessels without doubt from the meso- 

 blast. The gall-bladder is developed as a diverticulum from the he- 

 patic duct. The spleen, lymphatic, and thymus glands are developed 

 from the mesoblast : the thyroid partly also from the hypoblast, which 

 grows into it as a diverticulum from the fore-gut. 



THE RESPIRATORY APPARATUS. 



The lungs, at their first development, appear as small as tubercles, 

 or diverticula from the abdominal surface of the oesophagus. 



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 alimentary canal send off 

 secondary branches into the surrounding mesoblast, and these again give 



FIG. 494 illustrates the development of the respiratory organs. A, is the oesophagus of a chick 

 on the fourth day of incubation, with the rudiments of the trachea on the lung of the left side, 

 vie wed laterally; 1, the inferior wall of the oesophagus; 2, the upper tube of the same tube; 3, the 

 rudimentary lung; 4, the stomach. B. is the same object seen from below, so that both lungs are 

 visible, c, shows the tongue and respiratory organs of the embryo of a horse; 1, the tongue; 2, the 

 larynx; 3, the trachea; 4, the lungs viewed from the upper side. (After Rathke.) 



off tertiary branches, forming the air-cells. Thus we have the lungs 

 formed ; the epithelium lining their air-cells, bronchi, and trachea 

 being derived from the hypoblast, and all the rest of the lung-tissue, 

 nerves, lymphatics, and blood-vessels, cartilaginous rings, and muscular 

 fibres of the bronchi from the mesoblast. The diaphragm is early de- 

 veloped. 



THE GEXITO-URIKARY APPARATUS. 



The Wolffian bodies are organs peculiar to the embryonic state. 

 and may be regarded as temporary, rather than rudimental, kidneys ; 

 for although they seem to discharge the functions of these latter organs, 

 they are not developed into them. 



The Wolffian duct makes its appearance at an early stage in the his- 

 tory of the embryo, as a cord running longitudinally on each side in the 



