THE DEVELOPMENT OF THE RESPIRATORY SYSTEM. 363 



oesophagus, develop into the bronchi and their continuations the lungs. The 

 epithelial lining of the system is of course derived from the entoderm. The 

 various kinds of connective tissue are derived from the mesoderm, since the 

 anlage grows into the mesodermal tissue of the ventral mesentery. 



The Larynx. 



The opening from the gut into the respiratory tube becomes surrounded by 

 a U-shaped elevation thefurcula which lies in the floor of the pharynx with 

 its open end directed caudally. Toward the end of the first month each 

 side of the opening (aditus laryngis) becomes elevated, forming the arytenoid 

 ridge. From each of these a secondary elevation arises, forming the cunei- 

 form ridge. The arytenoid ridges come so close together that they practically 

 close the opening except at its cephalic side (Fig. 321). Along with the develop- 

 ment of these ridges the apical portion of the furcula becomes a distinct trans- 



Tuberculum impar 



^i Epiglottis 



I | f- Aryepiglottic ridge 



Arytenoid ridge 



--/- Cuneiform ridge 



p N .,/' . Aditus laryngis 



-. Cuneiform ridge 



FIG. 321. From a reconstruction of the larynx of a human embryo of 28 days. 

 Seen from above. Kallius. 



verse fold at the cephalic rim of the opening. This fold is the anlage of the 

 epiglottis. Laterally the epiglottic fold becomes continuous with the arytenoid 

 ridges, forming the aryepiglottic ridges (Fig. 321). 



During the fourth month a groove-like depression appears on the medial 

 side of each arytenoid ridge, gradually becomes deeper, and leaves on each side 

 of it a fold or lip which bounds the opening. The external lips those nearer 

 the pharynx form the superior or false vocal cords; the internal lips form the 

 true vocal cords. At the same time the opening into the larynx, which was 

 closed by the arytenoid ridges, is reestablished. The depression between the 

 vocal cords on each side becomes still deeper to form the ventricle, and a further 

 outgrowth from the ventricle produces the appendage of the ventricle (the laryn- 

 geal pouch). 



