chap. vi. j LUNGS OF VERTEBRATES. 239 



pentadactyle Yertebrata, or through the developmental 

 stages of a given individual. The earliest rudiment of 

 the lung is a single outgrowth (Fig. 101 ; A), which soon 

 divides at its blind end (B), while the unpaired 

 portion remains to form the tube (trachea) by 

 which the two sacs communicate with the oesophagus; 

 each swelling gives rise to primary (D), and these 

 to secondary (E) vesicles. 



This series of changes ceases at various points in 

 various forms, so that the lungs are smooth within in 

 Menobranchus, provided with a few simple ridges in 

 Siren, and with secondary as well as primary ridges in 

 Amphiuma. The internal network in which the 

 blood-vessels course is still more elaborately developed 

 in the frog, but the lungs are, when looked at from 

 without, apparently nothing more than simple sacs. 



The same is true of the lower Reptilia; but there 

 is this important advance, that the bronchus, or tube 

 which brings the air into the lungs, does not, as in the 

 frog, cease at the opening into the lung, but is con- 

 tinued into it, and gives off branches within it ; in 

 some chamseleons narrow blind outgrowths proceed 

 from the hinder end of the lung, and in Chelonians 

 and Crocodiles the common lung-chamber opens into a 

 number of pouch-like sacs. The lungs of the former, 

 like those of birds, are firmly attached on either side 

 of the vertebral column, and the dorsal surface is 

 marked by grooves which correspond in position to 

 those of the super] acent ribs. 



The lungs of Birds, in addition to their greater 

 internal complexity, are more particularly remarkable 

 for being continued into a number of air sacs, 

 whence prolongations are given off in the form of air 

 tubes and passages, which extend through all the 

 organs, including even the bones, of the body. These 

 air sacs play a very important part in the economy of 

 the bird, for they not only diminish its specific gravity, 



