LUNGS 



363 



trachea, and larynx, and this fact is supported by a study of their 

 comparative anatomy. 



Hollow outgrowths and buds arise from the endoderm lining 



PD 



Pit 



PD 



JS- 



- t 



m<sr 



FIG. 268. A, B, C, DIAGRAMS SHOWING THE MODE OF DEVELOPMENT OF THE 



LUNGS. 



b, bronchus ; PD, primitive alimentary tube ; S, S l , the lung-sacs, which are at 

 first unpaired ; t, trachea. 



the primary central cavity (" intrapulmonary bronchus ") of each 

 lung: these extend into the surrounding vascular mesoderm, which 



FIG. 269. DIAGRAMS ILLUSTRATING THE BUDDING OF THE BRONCHI IN THE 

 DEVELOPING LUNG OF EMYS. (After Fanny Moser.) 



A, buds from the intrapulmonary bronchus are shown extending into the lung- 

 wall. B. The intrapulmonary bronchus, which still forms a narrow tube 

 extending through the whole length of the lung, has given off a number of 

 primary pulmonary vesicles extending into the thick wall of the latter. C. 

 By a marked thinning-out of the lung-wall, the vesicles form chambers, of 

 which four dorsal (d) and- four ventral (v) are visible, each of which has 

 developed secondary buds (JV) ; the bronchus itself has become enlarged 

 and given rise to a terminal chamber (EK). D. Lung of adult. The 

 chambers have become enlarged and are separated from one another merely 

 by narrow septa, corresponding to the reduced lung- walls : the secondary 

 buds have given rise to buds of a third order, which form the lung-crypts, 

 and the terminal chamber (EK) is enlarged. 



gives rise to muscular fibres and connective tissue, and thus a 

 branched system of cavities communicating with the bronchi is 



