THE SYRINX OF GALLUS DOMESTICUS 185 



Along the dorsal surface in this region the condensation is no 

 thicker than in the preceding stages, but it is much thicker lateral 

 and ventral to the bifurcation (fig. 7). It also extends between 

 the bronchi, just caudad to their union with the trachea. In a 

 164 hour embryo, at the points in the lateral bronchial walls 

 where the above mentioned evaginations of the epithelium occur, 

 the aggregated mesenchymal cells arrange themselves into very 

 compact areas. These areas appear quite round in cross section. 

 In the center of each area are several unmodified cells which are 

 immediately surrounded by others of the same type concen- 

 trically placed. This arrangement of mesenchymal cells is the 

 beginning of the anlagen of the first bronchial half-rings which, 

 of all the skeletal elements, are the first to appear. This anlage 

 extends from the ventro-lateral to the dorso-lateral parts of each 

 bronchus, but appears best developed in its middle portion. 

 These anlagen are more marked in a 176 hour embryo and are 

 represented in figure 7, as two rounded and elongated masses 

 projecting dorso-ventrally around the lateral walls of the epi- 

 thelial tubes of the bronchi a httle below the tracheal bifurcation. 

 Twelve hours later the mesenchymal cells just cephalad to thes3 

 anlagen for the first bronchial half-rings become arranged so as 

 to form two other areas of aggregated cells similar to the ones just 

 described. These areas are the anlagen of the fourth intermedi- 

 ate syringeal cartilages, which at this stage he very near those 

 of the first bronchial half-rings. 



At 200 hours the first half-rings, and the fourth intermediate 

 cartilages stand out much more prominently, since the mese- 

 chyma immediately surrounding them is now less dense (fig. 

 13). The fourth intermediate syringeal cartilages differ from the 

 first half-rings only in extending farther ventrally. The anlage 

 of the pessulus appears very faintly in the mesenchyma between 

 the bronchi, and slightly caudad to the tracheal bifurcation. It 

 occupies the ventral three-fifths of the distance from the ventral 

 to the dorsal walls of the respiratory tube in the region of its 

 bifurcation. In structure it does not appear different from the 

 above described anlagen, but it is much larger than either of 

 them. A V-shaped mass of condensed mesenchyma extends from 



