ISyrinc <>j Futti. 299 



ill tlic more iloisal ic^^ion. On the left si<k' tlie dorsal dilatation 

 jtasses aronnd the bronehus, and iy approaching the nu-niVji-ana ex- 

 terna ; other air spaces are also present, coming towards the 

 lateral walls of the svrinx. (PI. XVlll.. tig. 2.) 



7. Sixteen days. — The chief progressive changes that have taken 

 place at the close of the sixteenth day have to do with the expansion 

 of the syringeal air sacs. (Pis. XXI., XX 11., and XXIIL, figs. 

 11-lG.j Very little change has occurred in the supporting framework. 

 The ventral tiiangular plate lias, fused to its sides, only the extremi- 

 ties of the last tracheal ring, and one end of the penultimate, the 

 other end. usually the right one, having separated off frou) the plate. 

 (PI. XX.. fig. 7.) The apex of the dorsal plate has become more 

 acute, and is surrounded, as before, by the free ends of the third, 

 fourth and tifth-last rings, the ends of the fifth ring nt)\v tending 

 to turn downwards, towards the apex of the plate. Much varia- 

 tion and irregularity is at times seen at this stage, and in later 

 stages, in regard to the fused or un fused condition of the dorsal 

 end of the above rings. The ends of the fifth-last may, or may not, 

 be fused. The different rings may be free from each other, or par- 

 tially fused, the tendency being, however, towards fusion of these 

 three rings in this dorsal region. In one case the fifth ring was 

 fused on one side with that of the sixth. 



As to the membranes, owing to the growth of the air sacs, they all 

 now take on their characteristic structure of three layers, and are 

 set between an air cavity on either side of them; .s<» that eventually, 

 in the adult, this probably becomes a condition of their vibration 

 during the alternate expansion and contraction of these air spaces, 

 as the air goes in and out. The membrana semilunaris is now very 

 well marked, and extends along the whole cranial border of the 

 pessulus. The tissue of the membranae externae is more reticular, 

 and their walls become thinner as the air sacs push into them. 

 (PI. XXTII.. fig. 15.) The membranae internae are now completely 

 formed. Their walls are thinner, owing to the reduction of the 

 mesoblastic tissue. To aVjout the middle of the left membrane the 

 l)ronchidesmus is attached. Little change has taken place in the 

 memljranae tracheales, except that its tissue has become more re- 

 ticular, the middle rings smaller, and the membrane as a whole 

 thinner. 



Syringeal air sacs. — As this is the stage at which the air sacs 

 take up their characteristic relation to the membranes, their origin, 

 extent and position will now be somewhat fully described. (Pis. 

 XXL. XXIT. and XXIIL. figs. 11-16.) 



