DEVELOPMENT. ; 817 



THE RESPIRATORY APPARATUS. 



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



Fig. 512 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, 

 viewed laterally; 1, the inferior wall of the oesophagus; 2, the upper portion 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.) 



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

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

 veloped. 



THE GrENlTO-URINARY 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 mass of mesoblast, which lies just externally to the intermediate cell- 

 mass (ung, fig. 513). This cord, at first solid, becomes gradually hol- 

 lowed out to form a tube (Wolffian) which sinks down till it projects 

 beneath the lining membrane into the pleuro-peritoneal cavity. 



The primitive tube thus formed sends off secondary diverticula at 



frequent intervals which grow into the surrounding mesoblast: tufts of 



vessels grow into the blind ends of these tubes, invaginating them and 



producing Malpighian bodies very similar in appearance to those of the 



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