Syrinx of Foai. 293 



is jHcoiiiplislud liv tliu upjiTowtli of the iiitLTtlaviiular sac^i from 

 the luii<^- into the iiiti'rl)roTi(.'lLial region. The steJii of thi' iiilcr- 

 chivifular sac iiiid tliat of tlie anterior thoracic air sacs, are tlie 

 hifurcatioiis of the main stem of tlio third entobronchus of the 

 huiu\ Tiie iiiterl)ronchial sacs are not symmetrical, the huge sul)- 

 pessuhir sac bein^' derived from the right side, and in contact Avith 

 botii membranae internae. As a result, there is a three-layered band 

 of tissue gradually developed, separatin'j: tlie air-spaces of the right 

 and left sides, and tending to connect obli(iuely the two membranes. 

 This band of tissue ])Ccomes the bronchidesmus, which, in the adult, 

 is tough and fibrous. The membranae traclieales, oi' the two lateral 

 walls of the upper part of the syrinx, have the last five or six 

 ti-acheal rings embedded in their tissue, and even up to the time 

 of hatching, tliese walls remain tliiek. After hatching, howevei-, 

 when the eml)edded rings rapidly l)egin to flatten, the Avails be- 

 come distinctly meml)raneous and tough, and finally constitute in 

 the adult tlu' nit^iihranae tracheales. 



It lias already been mentioned wliat an intimate relationship 

 comes to exist between the membranae internae and interl)ioncliial 

 ail' sacs. A similar relationshi]i comes al)out in reganl to the mem- 

 branae externae and tracheales. The stem of the interclavicular 

 sac not only gives off the interbronchial and subpessular sacs, but 

 also gives oft' lar<re venti-o-lateral and dorso-lateral sacs. The veii- 

 tral sac expands over the bronchi and the triangidar plate into 

 a large sac, Avhich gradually becomes closely applied to the ventro- 

 lateral half of tlie membranae externae and tracheales. The dorsal 

 sac expands around the dorsal surface of the bronchus and triangu- 

 lai- plate, giving oft' several dilatations, one large one coming to 

 occupy the dorso-latei'al half of the membranae exteriiae and 

 tracheales. 



Thus the whole syrinx liecomes enveloped in air sacs, and this 

 fact, together with the ]toor develoi)ment of the syringeal muscles, is 

 suggestive of tlie cause of vibration of the syringeal membranes. 

 (Pis. XVllI. and XIX., figs. 1, 2, 3.) 



The syringeal muscles luake their apjiearance at alx^ut the end 

 of the second week of incubation. Avhere tliey are seen developing 

 beneath the tracheo-clavicular muscles. A dorsal and a ventral 

 pair develop, but they do not extend down as far as the syrinx, stop- 

 ping short at about the eleventh or twelfth tracheal ring from the 

 caudal end. 



Such is the general survey of the developing organ. A more de- 

 tailed descri)>tion of various stages will how be given, siiowing the 

 development of the several parts, in order of time. 



