298 HAND-BOOK OF PHYSIOLOGY. 



trunks for the conveyance of the blood to the heart. The inner portion 

 of the cones consists of a number of solid cylindrical masses of cells, 

 derived probably from the hypoblast, which become gradually hollowed 

 by 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 

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

 hepatic 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. 



DEVELOPMENT OF THE KESPIRATOEY APPAEATUS. 



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

 diverticular from the abdominal surface of the oesophagus. 



The two diverticular 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 



FIG. 459 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 wall of the same tube; 3, the rudiment- 

 ary 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.) 



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, nerves, 

 lymphatics, and blood-vessels, cartilaginous rings, and muscular fibres of 

 the bronchi from the mesoblast. The diaphragm is early developed. 



THE WOLFFIAN BODIES, URINARY APPARATUS, AND SEXUAL ORGANS. 



The Wolfifian 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. - 



