ALIMENTARY CANAL. 



765 



iv 



paired diverticula, or as a single diverticulum ; and as to whether the 

 rudiments of the lungs are established 

 before those of the trachea. If the above 

 account is correct it would appear that 

 any of these positions might be main- 

 tained. Phylogenetically interpreted the 

 ontogeny of the lungs appears however 

 to imply that this organ was first an 

 unpaired structure and has become 

 secondarily paired, and that the trachea 

 was relatively late in appearing. 



The further development of the 

 lungs is at first, in the higher types 

 at any rate, essentially similar to 

 that of a racemose gland. From 

 each primitive diverticulum nu- 

 merous branches are given off. 

 In Aves and Mammalia (fig. 355) 

 they are mainly confined to the 

 dorsal and lateral parts. These 

 branches penetrate into the sur- 

 rounding mesoblast and continue 

 to give rise to secondary and 

 tertiary branches. In the meso- 

 blast around them numerous ca- 

 pillaries make their appearance, and the further growth of the 

 bronchial tubes is supposed by Boll to be due to the mutual 

 interaction of the hitherto passive mesoblast and of the hypo- 

 blast. 



-PC 



FIG. 419. SECTION THROUGH 

 THE CARDIAC REGION OF AN EMBRYO 

 OF LACERTA MURALIS OF 9 MM. TO 

 SHEW THE MODE OF FORMATION OF 

 THE PERICARDIAL CAVITY. 



ht. heart ; PC, pericardial cavity ; 

 al. alimentary tract; Ig. lung; /. 

 liver; //. body cavity; md. open 

 end of Mullerian duct; wd, Wolffian 

 duct ; vc. vena cava inferior ; ao, 

 aorta ; ch, notochord ; me. medullary 

 cord. 



The further changes in the lungs vary somewhat in the different forms. 



The air sacks are the most characteristic structures of the avian lung. 

 They are essentially the dilated ends of the primitive diverticula or of their 

 main branches. 



In Mammalia (Kolliker, No. 298) the ends of the bronchial tubes become 

 dilated into vesicles, which may be called the primary air-cells. At first, 

 owing to their development at the ends of the bronchial branches, these are 

 confined to the surface of the lungs. At a later period the primary air-cells 

 divide each into two or three parts, and give rise to secondary air-cells, while 

 at the same time the smallest bronchial tubes, which continue all the while 

 to divide, give rise at all points to fresh air-cells. Finally the bronchial 

 tubes cease to become more branched, and the air-cells belonging to each 

 minute lobe come in their further growth to open into a common chamber. 



