166 LUNGS 



The lung-s. The lungs begin to develop during the third week from the ventral 

 part of the pharynx at its junction with the oesophagus (fig. 199, Lg). The lung- 

 rudiment is at first single and median, and takes the form of an elongated vertical 

 diverticulum of the fore-gut, communicating freely with that tube, and of course 

 lined by entoderm. Soon the diverticulum sprouts out at its lower extremity in the 

 form of two tubes which grow downwards on either side of the heart, into a mass 

 of mesodermic tissue, which keeps pace in its growth with the lung-rudiment, and 

 from which the connective tissues of the future lung become ultimately developed. 

 The extremities of the tubes in question are early seen to be dilated and lobulated 

 (fig. 208), three lobules being present on the right tube and two on the left. The 

 division of the lungs into their lobes is thus early indicated. 



The further outgrowth of the tabulations produces the rudiments of the principal branches 

 of the bronchi, one for each future pulmonary lobe. Each of these branches then gradually 

 progresses in growth, giving off as it proceeds diverticula which form the secondary bronchi, 

 and these again giving off others until the whole complicated bronchial ramification is 

 eventually produced. Like the first sprouts from the median diverticulum, all the secondary and 

 other sprouts are dilated at their termination, and have a lobulated aspect (fig. 208, B ; fig. 220, 

 p. 174 ; fig. 268, p. 214). This is due to the fact that they are undergoing a further division or 

 sprouting. This process goes on until the sixth month of intra-uterine life, by which time all 

 the dilated ends of the growing and sprouting tubes have reached the surface of the lung. 

 These dilated extremities, which now appear grouped together, and apparently springing several 

 from a common tube, form the infundibula, but their walls are not at first beset with air-cells. 

 The formation of these takes place when the bronchial ramification is completed (sixth month, 

 Kolliker), as small, closely set, pouch-like protrusions of the walls of the infundibula, and of 

 the terminal bronchial tubes. 



There has been much difference of opinion as to whether the branching described above is to be 

 regarded as monopodial or dichotomous (i.e. whether the bronchial tree is developed by the growth 

 a chief stem from which secondary side-branches are given off, or by the continuous dichotomous 

 division of the terminal buds. The truth seems to lie with His, who described for the human 

 embryo a monopodial division of the primary stems, and a dichotomous division for the secondary 

 branches. This is the view expressed in the most recent paper on the subject by Flint ' ; 

 though he found that the secondary branches may also develop by monopodial division for two 

 or three generations. 



The trachea and larynx are formed by a separation from the oesophagus 

 of the original median diverticulum, from the lower angles of which the bronchial 

 rudiments have sprung, the separation commencing below and leaving a relatively 

 small connexion between the two tubes above : this connexion is the rudimentary 

 glottis. As development advances, both the tracheo-laryngeal and the oesophageal 

 tubes lengthen, the latter relatively more than the former, so that the lung- 

 rudiments no longer lie, as was the case at first, in front of and on either side of 

 the stomach, but extend downwards somewhat short of that organ, separated 

 from one another by the oesophagus behind and the heart and pericardium in 

 front. As they thus grow backwards with the lengthening of the trachea, the 

 lung-rudiments project into the anterior part of the body-cavity or ccelom (dorsal 

 portion), and receive a covering from its lining membrane, at first only below 

 and on the external surface, but subsequently on the internal aspect, so as to 

 separate them from the oesophagus (fig. 220). The portions of the body-cavity 

 into which the lungs project become shut off from the remainder on the 

 formation of the diaphragm and pericardium, and form the pleurae. 



The rudiment of the larynx appears about the twenty-fifth day, before the 

 trachea is separated off from the median diverticulum, in the form of two lateral 

 swellings (Arytdnoidwulste, Kallius), which lie behind the fourth visceral pouches, 

 and here compress the fore-gut in a sagittal direction. They possibly represent 



1 Amer. Journ. of Anat. v. 1906. On this subject see also a recent paper on the comparative 

 embryology of the lung by Moser, Arch. f. mikr. Anat. Ixiii. 1903. 





