Syriu.r of FoxvL 297 



G. Fifteen days. — Tiiis .stafj^e of (levelitpinent is of coiisiderahle 

 importanee, since, by this time, tiu- foundation of all the main 

 stiuetures of the syrinx has been laid. (PI. XVIII., fig. 27.) 



The supporting framework, with all its elements, is dehnitely 

 formed, all the syringeal membranes are to some extent indicated, 

 the syringeal muscles are developing, and lastly, the syringeal air 

 sacs begin to show intimate relationship with the syrinx as a whole. 

 As to the general enveloping mesoblastic tissue, it is now losing its 

 dense character, and gradualh' changing into a reticular meshwork, 

 but the tissue closely around the lower tracheal rings is still dense, 

 and is made up of several layers of jflattened cells. 



Supporting framework. — A few changes have taken place. Pre- 

 vious to this stage, the last four tracheal rings have been fused 

 with the sides and apex of the ventral triangular plate; but now, at 

 this stage, there is the beginning of a change, which later results 

 in the separation of all but the last ring from the plate. (Pis. XIX., 

 XX., figs. 4 and 8.) 



This goes on gradually, and reaches completion some time after 

 the hatched condition. There is variation in the actual time of 

 separation, l)ut at the end of the fifteenth day the apex of the ven- 

 tral plate is no longer intimately fused with the fourth-last ring. 

 Again, the ends of the third-last now articulate with each side of 

 the apex, so that only the last two rings remain fused with the sides 

 of the plate. (PI. XIX., fig. 6.) 



It may also be noticed that the ventral ends of the first two 

 bronchial semi-rings are now- more definitely fused. The smallei' 

 size of the dorsal triangular plate, as compared with the ventral 

 one, is now seen. Also, it is not directly fused with any of the 

 tracheo-bronchial rings. As before, its lateral angles, articulate 

 with the first bronchial semi-ring, its sides with the ends of the 

 last two tracheal rings, and its apex articulates with the ante-pen- 

 ultimate ring, whose ends are now fusing. 



On the left side, the fourth-last ring fuses at its extremity with 

 the fifth-last ring, just before the extremities come close together in 

 the mid-line. 



Syringeal membranes. (PI. XV'llI., fig. 2.) — At tlie close of 

 the fifteenth day, the raembrana semilunaris first begins to appear, 

 capping the more ventral region of the pessular rod. It is wedge- 

 shaped, and simply consists of a mesoblastic interior, covered by the 

 liypoblastic epithelium. The unattached edge proj^ects slightly into 

 the lumen of the tracheal extremity. The membranae externae be- 



