.Mammalia: Visceral Systems 



619 



the two bronchi) endodermal pulmonary outgrowth from the ventral 

 wall of the embryonic pharynx. But the expansion is effected in three 

 quite different ways. The fully developed reptilian lung, as viewed in 

 ordinary dissection, appears as a single large sac, usually of simple 

 external form, more or less subdivided internally by incomplete parti- 

 tions, which seem to arise as inward folds from the walls of the sac 

 (Fig. 473D). In some cases the folds, projecting only slightly, intersect 

 one another to produce shallow pockets so that the inner surface of the 

 sac resembles a piece of honeycomb whose compartments have been 

 laid open by cutting across the walls. Lungs of this simple structure, 

 resembling lungs of frogs and toads, occur in Sphenodon and many 

 lizards. More commonly the interior is divided into spaces of varying 

 size and somewhat irregular arrangement, each opening freely into 

 others. The extent to which this subdividing is carried and the ar- 

 rangement of the spaces varies greatly in the several groups of reptiles. 

 It is most elaborate in Crocodilia and Chelonia. The internal surfaces 

 of the spaces are not smooth but are sacculated — i.e., beset with 

 shallow pockets or alveoli resembling those which give the honeycomb 

 appearance to the inner surface of the simpler lungs of anurans and 

 some lizards. The region of the bronchus external to the lung is, like 



Km. IT.'?. Diagrams illustrating the budding <>l I he bronchus in the developing 

 lung of a turtle, Emys. (A) Buds from the intrapulmonary bronchus extend into 

 the lung-wall. (B) The bronchus has given off several buds (primary pulmonary 

 vesicles) extending into the thick wall of the lung. (C) The vesicles enlarge, the 

 lung-wall becoming relatively thinner, and form chambers, of which four dorsal (d) 

 and four ventral (v) are visible, each of which has developed secondary buds (N) ; 

 the posterior end of the bronchus has enlarged to form a terminal chamber (EK). 

 (D) Lung of adult. The several chambers have become enlarged and are separated 

 from one another merely by thin septums, consisting of the reduced lung-walls: 

 the secondary vesicles have given rise to buds of a third order, which form the 

 lung-crypts (alveoli). (After Moser. From Wiedersheim . "Comparative Anatomy 

 of Vertebrates." By permission of The Macmillan Company, publishers.) 



