162 EMBBYOLOGY OF THE LOWER VEETEBEATES ch. 



being, as is so often the case in the development of hollow organs, 

 that the rudiment is at first solid and the cavity appears secondarily 

 in its interior. This is the case in various anurous amphibians and 

 in Lepidosiren and Protopterus. 



It has been indicated that the lung is primarily a ventrally 

 placed pocket of the pharyngeal wall, that is to say its wall is a 

 portion of splanchnopleure. It follows that the cavity of the lung 

 is lined by endoderm while its outer layers (connective tissue, blood- 

 vessels, muscles, etc.) are composed of splanchnic mesoderm. 



As regards the further development of the lung, the main steps 

 are concerned with its respiratory function and have to do with the 

 increase of the respiratory surface. In such an animal as the Newt, 

 where the lung retains a relatively primitive condition, the endo- 

 dermal lining grows equally as the organ increases in size, so that 

 even in the adult the lung has the form of a simple sac with smooth 

 endodermal lining. In a Frog or a Lizard, however, growth activity 

 is specially marked at particular spots so that at these spots the 

 endoderm forms outward bulgings into the covering of splanchnic 

 mesoderm. 



In these Sauropsida in which the pulmonary apparatus reaches 

 its highest degree of evolution (Tortoises, Turtles, Crocodiles and 

 Birds, in an ascending series) these pockets of the endodermal 

 lining become more and more extensive, and more and more com- 

 plicated, so as to give rise to a thick spongy mass, which forms the 

 bulk of the lung, surrounding the now relatively small clear central 

 space. The latter, forming as it does an apparent continuation 

 of the bronchus or paired portion of the trachea, is spoken of as 

 the intrapulmonary bronchus. Further the respiratory function 

 becomes concentrated towards, the terminal portions of the pockets, 

 their proximal portions forming simply conducting channels — 

 branches of the intrapulmonary bronchus. 



In the Chameleons, towards the end of development, a number 

 of the endoderm outgrowths bulge out beyond the general level of 

 the surface of the lung upon its ventral side. These persist in the 

 adult as large diverticula which when the animal blows itself out 

 are inflated with air. In the embryos of Birds similar outgrowths 

 make their appearance, four from each lung, but in this case as 

 development goes on the outgrowths continue to increase in size 

 and form the characteristic air-sacs of the adult bird. 



The Lung of Bieds. — As the Birds, in correlation with the 

 intensely active metabolism as indicated e.g. by their high body 

 temperature, stand preeminent amongst Vertebrates in the hio-h 

 stage of evolution which has been reached by their lung, the onto- 

 genetic development of this organ will be followed out in a little 

 more detail (Moser, 1902; Juillet, 1912). 



In the Fowl the pulmonary diverticulum of the pharyngeal floor 

 makes its appearance about the beginning of the third day. By the 

 end of this day the rudiment is bifurcated at its hind end, each lobe 



