THE ORGANS OF THE INNER GERM-LAYER. 



323 



come to lie more and more at the right and left of the heart. With 

 their ingrowth into the cavities of the chest (fig. 314 brh), they push 

 before them the serous lining of the latter, and thus acquire their 

 pleural covering (the pleura pulmonalis, or the visceral layer of the 

 pleura). 



During the second stage the organ, which up to this time has the 

 typical structure of a botryoidal gland, assumes the characteristic 

 pulmonary structure. The metamorphosis begins in Man, as 

 KOLLIKER states, in the sixth month, and comes to a close in the 

 last month of pregnancy. There now arise close together on the 

 fine terminal tu- 

 bules of the bron- 

 chial tree, on the 

 alveolar passages, 

 and on their ter- 

 minal vesicular 

 enlarge in ents, 

 very numerous 

 small e v a g i n a- 

 tions. But in dis- 

 tinction from the 

 earlier ones, these 

 are not constricted 



Fig. 183. View of a reconstruction of the fundament of the lungs 

 of a human embryo (N of H:s) older than that of fig. 182. 

 AftirH:s. Magnified 50 diameters. 



Ap, Aiteria pulmonalis ; Lr, trachea ; sp, oesophagus ; Z6, 'pulmonary 

 vesicle in process of division ; 0, upper lobe of the right lung 

 with an eparterial bronchus leading to it ; ]if, V, middle and 

 lower lobes of the right lung; 0', upper lobe of the left lung 

 with hyparterial bronchus leading to it ; U l , lower lobe of the 

 left lung. 



off from their 



source of origin, 



but communicate 



with the latter 



by means of wide 



orifices, and thus 



constitute the air-cells or pulmonary alveoli. Their size is only a 



third or fourth as great in the embryo as in the adult ; from this 



KOLLIKER concludes that the increase in the volume of the lung 



from birth up to complete development of the body is to be attributed 



exclusively to the enlargement of the vesicular elements which exist 



in the embryo. 



The epithelial lining of the lung is variously modified in different 

 regions during development. In the whole bronchial tree the 

 epithelial cells increase in height, acquire in part a cylindrical, in 

 part a cubical form, and from the fourth month onward (KOLLIKER) 

 have their free surfaces covered with cilia. In the air-sacs, on the 

 contrary, the cells, which are arranged in a single layer, become 



