298 A. (J. V. Tijmms: 



tween the tracheal I'injrs and the bronchial semi-rings are now 

 larger in size, and somewhat deflected into the lumen of the syrinx. 

 Their walls are still thick and dense in structure. 



The membranae internae are now developing from the inner 

 walls of the bronchi, between the free end of the bronchial semi- 

 rings. At first, the mesoblastic tissue between the bronchi is one 

 continuous sheet, but at the close of the fifteenth day the inter- 

 bronchial air sacs have so extended upwards as to reach almost to 

 the pessulus; hence the mesoblastic tissue is divided into two layers. 

 In this way the membranae internae first begin to form. Their walls 

 have three layers, and the mesoblastic layer is as yet of considerable 

 thickness. As yet the right air sac has not completed the splitting 

 of the mesoblast in the dorsal region, where the two membranes 

 have not become differentiated. The hypoblastic epithelium of the 

 membranes is thickened, and contains spaces, so that it tends to 

 become folded. With the pushing up of the two interbronchial air 

 sacs, the bronchidesmus becomes formed at this stage. It consists 

 of the two thin walls of the right and left air sacs, with a strand 

 of mesoblastic tissue between them, and continuous with the meso- 

 blastic walls of the membranae internae. The intimate association 

 of these three layers constitutes the bronchidesmus, and, owing to 

 the greater and asymmetrical development of the right sac, the mem- 

 brane is directed obliquely between the bronchi. 



The membranae tracheales do npt become membraneous in struc- 

 ture until after hatching, and hence at this stage their walls are 

 thick, and the tracheal rings embedded in them have only just begun 

 to flatten. There is also to be seen a denser tissue, connecting the 

 rings. 



Muscles of the syrinx (PI. XVIII., fig. 2.)— By this stage, the 

 tjterno-tracheales and tracheo-cjavieulai- muscles are well marked, 

 lying close to the ventral and lateial sides of the trachea. The 

 trachoo-clavicular leave the lattei- walls of tliu tracli^ea just above 

 the region of the eleventh and twelfth-last tracheal rings. Between 

 these Tnuecles and the trachea the syriiigt'al muscles are developing 

 and extending down a short distance towards tlu' syiinx. They do 

 not reach the syrinx, and, as yet, they are not clcaily differentiated 

 into a dftrsal and a ventral pair. 



Syring^al air sacs. — The syringeal air sacs have now become mucii 

 expanded, both between the bronchi, and also to the lateral side 

 of the syrinx. On the right side th<' suV)-pessular air sac has been 

 given off. and thus the membranae internae are now <lefined, except 



