366 



TEXT-BOOK OF EMBRYOLOGY. 



destined to give rise to the connective tissue, includng the cartilage, of the adult 

 trachea (Figs. 284 and 320). The development of the tracheal rings is very 

 similar to that of the laryngeal cartilages. During the eighth or ninth week con- 

 densations appear in the mesenchyme, which are later transformed into hyalin 

 cartilage. The rings are not complete but remain open on the dorsal side. At 

 birth the trachea is collapsed, the ventral side being concave and the dorsal ends 

 of each ring being in contact. After respiration begins it is dilated and becomes 

 more or less rigid. Ossification of the tracheal rings begins in the male at the 

 age of about forty years, in the female at about sixty. The glands of the 

 trachea represent evaginations from the epithelial linings. 



The Lungs. 



As has been stated (p. 362), the caudal end of the original tube evaginates 

 to form two hollow buds which are the beginnings of the two lungs (Fig. 324). 

 The evagination takes place soon after or even along with the separation of the 

 lung groove from the gut. The right bud soon gives rise to three secondary 



Aorta 



Upper limb bud 

 (Esophagus 



Body cavity 

 Pericardial cavity 



FIG. 324. Transverse section of a 14 mm. pig embryo, at the level of the upper limb buds, 

 showing especially the two bronchi. 



buds, the forerunners of the three lobes of the right lung. The left bud gives 

 rise to tyyo secondary buds, the forerunners of the two lobes of the left lung 

 ( Fi g- 3 2 5)- The primary buds may be said to represent the two bronchi arising 

 from the trachea, the five secondary buds to represent the bronchial rami 

 which extend into the five lobes of the lungs. Successive evaginations from 

 each of the five buds take place and form an extensive arborization for each 

 lobe (Figs. 326 and 327). 



