DEVELOPMENT OF LUNGS AND BUOYANCY STRUCTURES 649 



two major bronchi. It is an elaborate structure, consisting of a number of folds 

 of the mucous membrane together with an enlargement of this particular area. 

 This structure is known as the syrinx (fig. 308A). The morphological struc- 

 ture of the syrinx varies from species to species. The ultimate position of the 

 bird's lung in the body is shown in figure 308B. 



5) Basic Cellular Composition of the Trachea, Lungs, and Air Sacs. It is 

 obvious from the description above that the entire lining tissue and the res- 

 piratory membrane of the bird's respiratory and air-sac system are derived 

 from the original entodermal evagination, whereas the muscle, connective, and 

 other tissues are formed from the surrounding mesenchyme. 



c. Development of Lungs in the Mammal 



1) Origin of the Lung Rudiment. The first indication of the appearance 

 of the lungs in the pig and human embryo is the formation of a midventral 

 trough or furrow in the entoderm of the pharynx, the laryngotracheal groove. 

 This groove forms immediately posterior to the fourth branchial (visceral) 

 pouch, approximately at the stage of 3 to 4 mm. in both pig and human. In 

 the human, about the fourth week, and 3-mm. pig, the laryngotracheal groove 

 deepens, and its posterior end gradually forms a blind, finger-like pouch which 

 creeps posteriorly below the esophageal area as a separate structure (fig. 

 309A). Thus, the original laryngotracheal groove is restricted to the cephalic 

 end of the developing lung rudiment, where it forms a slit-like orifice in the 

 midventral floor of the pharynx at about the level of the fifth visceral (i.e., 

 third branchial) arch. 



2) Formation of the Bronchi. As the caudal end of the original lung rudi- 

 ment grows caudad, it soon bifurcates into left and right bronchial stems as 

 shown in figure 309B. Each primary or stem bronchus is slightly enlarged at 

 the distal end. As the stem bronchi of the right and left lung buds continue 

 to grow distally, evaginations or secondary bronchi arise progressively from 

 the primary bronchi as indicated in figure 309C-E. While this statement 

 holds true for the human embryo, the apical bronchus (i.e., eparterial bronchus 

 because this lobe of the lung comes to lie anterior to the pulmonary artery) 

 in the pig arises directly from the trachea as shown in figure 309D. Each of 

 these secondary bronchi forms the main bronchus for the upper and middle 

 lobes of the lungs (fig. 309D, E). From each lobular bronchus, other bronchial 

 buds arise progressively and dichotomously, with the result that the bronchial 

 system within each lobe of the lung becomes complex, simulating the branches 

 upon the limb of a tree. Considerable variation may exist in the formation of 

 the various bronchi in different individuals. 



3) Formation of the Respiratory Area of the Lung. This growth of bronchial 

 buds of the pulmonary tree continues during fetal life and for a considerable 

 time after birth. The large bronchi give rise to smaller bronchi, and, from the 

 latter, bronchioles of several orders originate. Finally, the terminal bronchioles 



