THE RESPIRATORY SYSTEM 



343 



diverticulum swells to form the lung anlage while the less expanded 

 anterior portion becomes the larynx and trachea. The lung anlage later 

 divides into two lateral buds which, by successive subdivision, gradually 

 assume the adult structure. (Fig. 286) 



The cartilages which support the larynx correspond exactly with 

 those which in aquatic vertebrates support the fourth and fifth branchial 

 arches. The muscles of these arches form the laryngeal muscles. Vocal 

 cords appear during the eleventh week. 



Beginning with the fifth week, the paired lung-buds branch in the 

 manner of a compound tubular gland. In this way, the entire lining of 



MOUTH INVAGINATION 

 lU. POUCHES 



TRACHEA 

 BRONCHUS- 



ANLASE OF UUNG 

 PHARYNX 



PRONEPHROS 



Fig. 287. — A-D Stages in the development of lungs in vertebrates. A is a hori- 

 zontal section of a salamander embryo showing the series of paired pouches which form 

 the gill slits after Goette. The last pair of pharyngeal pouches are the anlagen of the 

 lungs. Such evidence suggests that lungs may have arisen in phylogenesis from a pair 

 of gill pouches which failed to reach the surface. B and C are earlier and later stages 

 in the development of the lungs in an amphibian. D is a. cross section of the lung anlage 

 in a reptile, after Wiedersheim. (Redrawn from Ihle.) 



the lungs is derived from the pharyngeal endoderm. The connective 

 tissue develops from the surrounding mesenchyma. The splanchnic 

 mesoderm forms the serosa, which covers the lungs and Hues the chest 

 cavity. As the two lungs enlarge, they push laterally into the body cavity 

 and by their ventral extension nearly surround the heart, from which they 

 remain separated by the pericardium. With the development of the 

 diaphragm, the pleural cavity containing the lungs becomes separated 

 from the more posterior peritoneal cavity. According to Broman the 

 diaphragm arises from four sources, the septum transversum anterior to 

 the liver, the pleuroperitoneal membranes, the body-wall, and the dorsal 

 mesentery. 



The nasal passages of lower vertebrates, such as the dipnoi and 

 amphibia, develop from nasobuccal grooves similar to those seen in some 

 adult elasmobranchs. In the embryo an ectodermal groove extends from 



