DEVELOPMENT OF THE KESPIKATOKY APPAEATUS. 1101 



The Lung's. The lungs are developed from the two diverticula of the caudal end 

 of the median longitudinal groove and the mesodermal tissue into which these grow. 

 Originally single, this caudal end soon becomes bilobed and pouches out on each side into 

 two lateral diverticula, which represent the primitive bronchi and lungs. From the first 

 the right pulmonary diverticulum or vesicle is slightly the larger of the two. Both 

 diverticula elongate, and almost immediately undergo a subdivision the right into three 

 vesicles, and the left into two vesicles thus early indicating the three lobes of the right 

 lung and the two lobes of the left lung. As the primitive respiratory tube lies in the 

 median plane in the dorsal attachment of the septum transversum, the pulmonary diver- 

 ticula grow laterally and dorsally into the dorsal parietal recesses, that is into the future 

 pleural cavities, carrying before them a covering of mesoblast. From this rnesoblast are 

 derived the blood-vessels and other tissues which build up the lung, whilst the entodermal 

 cells which form the lining membrane of the primitive respiratory tube eventually 

 develop into the epithelial lining of the air-passages, and are embedded within the 

 surrounding mesoderm. The main entodermal subdivisions continue to branch and 

 re-branch, pushing their way into the pulmonary mesoblast, until the complete bronchial 

 tree is formed. 



The primary pulmonary diverticula increase in size and complexity as additional out- 

 growths arise by the subdivision of the enlarged terminal part of each diverticulum. 

 Their mode of subdivision is very characteristic, and from the first the various branches 

 are bulbous or flask-shaped at their extremities. These bifurcate, and although at first 

 the two main subdivisions appear, in each case, of equal importance, one grows out as the 

 continuation of the main bronchial stem the future hyparterial bronchus whilst the 

 other remains as a branch. When the ramification of the entodermal tubes into the 

 hmg-mesoderm is complete, the small terminal flask-shaped extremities of the various 

 branches represent the atria of the lung. 



This repeated bifurcation results, as just stated, in the formation of a main bronchus 

 which traverses the entire length of the lung, and into which numerous secondary 

 bronchi open. The latter, from the manner in which they arrange themselves around the 

 main stem of the pulmonary artery, are divided into dorsal and ventral. These alternate 

 with each other, and usually number four in each series ; not infrequently the third dorsal 

 bronchus fails to develop. In the left lung the first dorsal bronchus arises, not from the 

 ; main tube as on the right side, but from the first ventral bronchus an arrangement 

 which probably results from the fusion on the left side of the superior and middle lobes of 

 the left lung into one, namely, the so-called lobus superior of the adult left lung. 



The secondary bronchi elongate, and give rise to the tertiary bronchi, and these in 



turn to lesser bronchi, and so on down to the terminal bronchi, with their atria, air-sacs, 



i and air-cells of the lung-unit. At first the lung-unit is devoid of air-cells, but between 



the sixth month and full term the alveolar saccules and air-cells make their appearance 



on the alveolar ducts ; and it is thus clear that the epithelial lining of the entire system 



of bronchial subdivisions and ramifications is derived originally from the entodermal 



lining of the primitive foregut. By the close of the fourth month of foetal life the 



. columnar cells lining the trachea and bronchi have become ciliated. 



At first the diverticula of the respiratory tube are surrounded by thick masses of 

 mesoblastic tissue, but as development proceeds the latter fails to keep pace with the 

 former, and hence the mesoblastic tissue becomes greatly reduced in amount and in 

 i thickness. Coincidently, this mesoblast becomes vascularised, and thus rich plexuses of 

 blood-vessels come to surround the terminal divisions of the epithelial tubes an arrange- 

 | ment obviously adapted to the interchange of gases from air to blood and vice versa. 



The rudiments of the developing lungs grow dorsally on each side of the oesophagus 

 into the fissure-like portion of the coelom which occupies the thoracic region. They 

 .push before them the endothelial lining of the coelom, and thus come to acquire their 

 i covering of pulmonary pleura. By the development of the diaphragm and the peri- 

 cardium the pleural portions of the coelom become cut off from the peritoneal cavity and 

 from each other. 



