322 



EMBRYOLOGY. 



of the fourth and fifth visceral arches, whereas the cricoid and ary 

 tenoid cartilages, as well as the half -rings of the trachea, are 

 independent chondrifications in the mucous membrane. 



Two stages are recognisable in the metamorphosis of the primitive 

 lung-sacs of Man and Mammals. 



The first stage begins with the elongation of the sac, which is 

 attenuated at its origin from the trachea, but is enlarged at its 

 opposite or free end. At the same time in Man from the end of 

 the first month (His) it pushes out, in the manner of an alveolar 

 gland, hollow evaginations, which grow out into the thick connective- 

 tissue envelope and enlarge at their ends into little sacs. The first 

 bud-like outgrowths on the two sides of the body are not symmetrical 

 (fig. 182), because the left lung-sac jrroduces two, the right three bud-like 



enlargements. An im- 

 portant feature of the 

 architecture of the lungs 

 is thus established from 

 the beginning, namely, 

 the differentiation of the 

 right lung into three 

 chief lobes, and of the 

 left into two. 



The further budding 

 is distinctly dichotomous 

 (fig. 183). Ittakesplace 

 in the following way : 

 each terminal vesicle 



(primitive lung-vesicle), which is at first spheroidal, becomes flattened 

 and indented on the wall (Ib) which lies opposite its attachment. 

 Thus it becomes divided, as it were, into two new pulmonary vesicles, 

 each of which is then differentiated into a long stalk (lateral bronchus) 

 and a spherical enlargement. Inasmuch as such a process of budding 

 is kept up for a long time, in Man until the sixth month, there an- > 

 a complicated system of canals, the bronchial tree, which opens into 

 the trachea by means of a single main bronchial tube from either 

 side of the body, and the ultimate branches of which, becoming linn- 

 and finer, terminate in flask-shaped enlargements, the primitive 

 lung-vesicles. The latter are at first confined to the surface of the 

 lung, while the system of canals occupies its interior. 



During this budding the lungs as they increase in volume 

 continue to grow downwards into the thoracic cavities, and thereby 



P 



Pig. 182. View of a reconstruction of the fundament of 

 the lungs of a human embryo (7V of His) 10 mm. long, 

 neck measurement, after H s. 



Ir, Trachea ; br, right bronchus ; sp, oesophagus ; If, con- 

 nective-tissue envelope and serous membrane (pleura) 

 into which the epithelial fundament of the lung grows ; 

 0, M, U, fundaments of the upper, middle, and lower 

 lobes of the right lung ; 1 , U l , fundaments of the 

 upper and lower lobes of the left lung. 



