ALIMENT A R Y CA NA L. 



G29 



above formation of the lungs and trachea cannot be seen from the 

 surface. As the paired diverticula of the lungs grow backwards, the 

 mesoblast around them takes however the form of two lobes, into 

 which they gradually bore their way. 



There do not seem to be any essential difterences in the mode of 

 formation of the above structures in the 

 types so far observed, viz. Amphibia, 

 Aves and Mammalia. Write) s differ as 

 to whether the huigs first arise as paired 

 diverticula, or as a single diverticulum ; 

 and as to wliether the rudiments of the 

 lungs are estabhshed liefore those of the 

 trachea. If the above account is correct 

 it would appear that any of these posi- 

 tions might be n)aintainecl. Phylogene- 

 tically 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 ap- 

 pearing. 



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 numerous 

 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 surrounding mesoblast and con- 

 tinue to give rise to secondary and 

 tertiary branches. In the mesoblast 

 around them numerous capillaries 

 make their appearance, and the further growth of the bronchial 

 tubes is supposed by Boll to be due to the mutual inte: action of the 

 hitherto passive mesoblast and of the hypoblast. 



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



The air sacks are the most chai'acteristic structures of tlie 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 hronchial 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 blanched, and the 



Fig. 419. Section through the 

 cardiac region of an embryo of 

 Lacehta Muralis of 9 mm. to shew 

 the mode of formation of the 

 pericardial cavity. 



]it. heart ; jtc. pericardial cavity ; 

 al. alimentary tract ; Ix). lung ; /. 

 liver ; p;;. body cavity ; md. open 

 end of Mullerian duct ; \cd. Wolffian 

 duct ; vc. vena cava inferior ; ao. 

 aorta ; ch. uotocbord ; mc. medullary 

 cord. 



