i66 



THE ENTODEEMAL CANAL AND THE BODY CAVITIES 



late. The thyreoid cartilage is formed as two lateral plates, each of which has' two centers of 

 chondrification. These plates grow ventrad and fuse in the median line. 



The anlages of the cricoid and arytenoid cartilages are at first continuous. Later, sepa- 

 rate cartilage centers develop for the arytenoids. The cricoid is at first incomplete dorsad, 

 but eventually forms a complete ring. 

 The cricoid may therefore be regarded v; %/ 



as a modified tracheal ring. The cornic- 

 ulate cartilages represent separated por- 

 tions of the arytenoids. The cuneiform 

 cartilages are derived from the cartilage 

 of the epiglottis. 



pl.pk.e. 



pl.a.e. 



Fig. 172. — Entrance to larynx in a 15 to 16 

 mm. human embryo (from Kallius). X 15. t, 

 Tuberculum impar; p, pharyngo-epiglottic fold; 

 e, epiglottic fold; l.e, lateral part of epiglottis; 

 cun., cuneiform tubercle; corn., corniculate tu- 

 bercle. 



Fig. 173.— The larynx of 160 to 230 mm. 

 human fetuses (Soulie and Bardier). X 6. From 

 a dissection, b.l., Base of tongue; e., epiglottis; 

 f.i.a., interarytenoid fissure; o.L, orifice of larynx; 

 pl.a.e., plica ary-epiglottica; pl.pk.e., plica phar- 

 yngo-epiglottica; cun., cuneiform tubercle; corn., 

 corniculate tubercle. 



The Trachea. — This gradually elongates during development and its colum- 

 nar epithelium becomes cihated. Muscle fibers and the anlages of the cartilag- 



FiG. 174. — Ventral and dorsal views of the lungs from a human embryo of about 9 mm. (after Merkd). 

 Ap., Apical bronchus; Di, D2, etc., dorsal, Vi, V2, etc., ventral bronchi; Jc, infracardial bronchus. 



inous rings appear at 17 mm. The glands develop as ingrowths of the epith,elium 

 during the last five months of pregnancy. 



