364 



TEXT-BOOK OF EMBRYOLOGY. 



The mesodermal tissue external to the epithelium (entoderm) of the larynx 

 gives rise to the various kinds of connective tissue including the laryngeal 

 cartilages. By the end of the fourth week condensations appear in the mesen- 

 chymal tissue, which are the forerunners of the cartilages, but true cartilage 

 does not appear until the seventh week. The anlagen of the thyreoid cartilage 



Sup. hy. 



Sup. hy. 



Inf. hy. 



Thyr. 



FIG. 322. From reconstructions of the mesenchymal condensations which represent the hyoid and 

 thyreoid cartilages in an embryo of 40 days. A, Ventral view; B > lateral view from right. 

 Kallius. 



Inf.hy., Inferior (greater) horn of hyoid; Sup.hy., superior (lesser) horn of hyoid; Thyr., thyreoid. 

 The portions indicated by black lines represent chondrification centers. 



are two mesenchymal plates, one on each side, which are bilaterally sym- 

 metrical and correspond to the lateral parts of the adult cartilage (Fig. 322, A). 

 These plates gradually grow ventrally and unite and fuse in the midventral 

 line (Fig. 323). Two centers of chondrification appear in each plate (Fig. 322, -4,) 



Pharynx 



Muscle 



:-. Arytenoid cartilage 



j*---- Thyreoid cartilage 



Muscle 



FIG. 323. From a transverse section through the pharynx and larynx of a human 

 embryo of 48 mm. Nicolas. 



and enlarge until the entire plate is converted into cartilage, the middle part 

 becoming elastic in character, the rest hyalin. 



Originally the cephalic edge of each thyreoid plate is connected with the 

 inferior horn of the hyoid cartilage (Fig. 322, B). This connection is subse- 

 quently lost, but a remnant of the connecting cartilage persists as the triticeous 



