302 A. 0. V. Tyinncs: 



this traclicti-biunchiul juiictiijii is converted into a truly mem- 

 braneous chamber, whose lateral walls are made up of the mem- 

 branae tracheales and externac. The membranae tracheales, between 

 the time of hatching and the adult condition, undergo much change. 

 The last five tracheal rings embedded in it flatten into extremely thin 

 band-like vestiges, while the tissue between them Ijecomes thin and 

 tough, so that the whole constitutes the stout membi anae tracheales 

 of the adult. The membranae externae become the most distinctive 

 membranes, thin, yet strong, oval in outline, and stretched between 

 the last tracheal ring and the well-marked first ])ronchial semi- 

 ring. 



A similar change occurs in the membranae internae. The bron- 

 chidemus gradually assumes the fibrous condition of the adult. 

 As regai'ds the supporting framework, tlie changes in the last five 

 tracheal rings have been noted. The first two bronchial semi-rings 

 become the most developed of all the syringeal I'ings, and they are 

 curved so that their concavitv looks ci'anially, and opposes that of 

 the last tracheal ring. The dorsal and ventral plates considerably 

 thicken and Ijecome very prominent, and only the ventral ends of 

 the last tracheal ling fuse with it. Their l)asal angles, however, 

 articidatc with the other tracheal rings and first bronchial semi- 

 rings. 



In the older fowls, especiallv in the Tiiale liird, the pessulus be- 

 comes calcified, and may become partially ossified, not only along 

 the whole extent of the rod, but also in thi' lentral portions of the 

 ilorsal and ventral triangular plates. In addition to this, there is 

 a small centre of ossification in the ventral ends of the first bron- 

 <-hial semi-ring, just before it articulates with the basal angles of 

 the ventral triangular plate. Ossification also takes ])lace in many 

 of the tracheal rings above the syrinx. The syringeal muscles 

 change very little. There are the dorsal and ventral pairs, and they 

 do not reach the syrinx. Finally, with the change in the external 

 walls of the syrinx, the characteristic shape of the adidt syrinx is 

 assumed — namely, the lateral walls have approached one another, o 

 that the lateral width of the syrinx is less than tliat of the trachea, 

 but the dorso-ventral depth is somewhat gi(>ater. 



As before, the whole svrinx is enveloped by the syringeal air sacs 

 or diverticula of the interclaviiular air sacs. 



