Syrinx of Fowl. 301 



diviTticula in the lun^' i-cgioii. In tliis way are foiiiicd the recur- 

 leiit bi'aiiclie.s, lar^e ami .small, of the inteixlavicvilar sac. 



The venti'o-lateral and dorso-latoral syringeal sacs press close 

 against the membranae externae, tracheales an<l bronciius. They 

 iwv also in close contact with each othei', and oveidie the dorsal and 

 ventral triangular plates. Thus all surfaces of the syrinx — dorsal, 

 ventral, lateral and interbronchial — are embraced l)y air sacs. 



.S. Seventeen to twenty-one days. — The development of the syrinx 

 after the close of the sixteenth day is one of degree rather than 

 the laying down of any new structures, hence the remaining days of 

 incubation are taken together. There are a few changes in the sup- 

 porting framework. There is first a histological change. Tlie 

 hyaline or cartilaginous matrix is gradually laid down, and 

 the cartilaginous cells come to lie in their characteristic lacunae. 

 On hatching, the left ventral end of the penultimate ring ceases to 

 be fused with the ventral triangular plate, so that oidy the last ring- 

 is now fused to the plate. (PI. XX., fig. S.) Dorsally, the down- 

 turned ends of the fifth-last ring are bound to the apex of the plate 

 by tissue, which becomes fibrous in the adult. A slight flattening 

 has occurred in the rings embedded in the membranae tracheales, 

 the intermediate ones being small, and bound together by a connec- 

 tive tissue. Hence the membranae tracheales, although thinner, 

 arc- still far from being membraneous. (PI. XIX., fig. 2.) The 

 membranae internae and externae have also become much thinner. 

 The syringeal muscles are more strongly developed. There is now 

 the dorsal and the ventral pair, but they do not extend further down 

 the trachea than the twelfth or eleventh-last tracheal rings. The air 

 sacs have now so completely surrounded the syrinx as to separate 

 it off from adjacent structures, such as the oesophagus and large 

 vessels of the heart, whicli are in close relation to the tracheo-ln-on- 

 chial junction. 



Other than the increased size of the elements of the syrinx, thi.^ 

 comprises the development up to the time of hatching. 



V. — l'(h<f-(iiihr>/()ii ic (Jcvrlopiiiciit . 



The syrinx, being peculiar to birds as a vocal organ, shows con- 

 siderable development in the post -embryonic period. This accounts 

 for some marked differences between the adult and the hatched con- 

 dition. 



Since all the morphological structures of the syrinx have been laid 

 down during the embryonic period, the development in the post- 

 embryonic period is chiefly histological, but to such a degree that 



