192 JAY ARTHUR MYERS 



Extending caudad from the lateral borders of the shaft of the 

 pessulus in the 404 hour stage, and taking part m the formation 

 of the medial bronchial walls, a well marked strand of connective 

 tissue fibers is ssen. A large number of these fibers become con- 

 tinuous with, and enter into the formation of, the bronchidesmus, 

 which now stretches between the medial walls of the two primary 

 bronchi. 



On comparing figures 5 and 16 it will be seen that the tympanic 

 membranes have not as yet reached a very high stage of devel- 

 opment. From their structure and their sizs one would expect 

 them to be quite inefficient as \'ibrating membranes, and capable 

 of producing only very simple sounds. The simple sounds actu- 

 ally produced by the young chick, seem to verify this deduction. 



At the time of hatching the various structures described are 

 not essentially different, except for size, from those of the last 

 stage (404 hours) discussed. It thus appears evident that the 

 syrinx is very immature at the time of hatching. Between this 

 time and the adult stage, the four most caudal tracheal rings 

 unite, in the manner previously described, to form the tym- 

 panum; the pessulus and the tracheal rings cephalad to the sec- 

 ond become ossified; and finally, the tympanic membranes be- 

 come more like those of the adult. In addition to becoming 

 thinner, these membranes become more extensive and their cor- 

 responding fibers become more longitudinally arranged. 



6. Sum7nary of development 



The first indication of the respiratory tract was observed in a 

 68 hour embryo in which the laryngeo-tracheal groove and the 

 developing bronchi were present. The trachea is at first very 

 shprt and, like the bronchi, is composed of an epithelial tube and 

 a loose smTounding mesenchyma. The bronchi are relatively 

 much longer than the trachea in the beginning. Owing, however, 

 to the rapid development of the neck, the trachea lengthens very 

 rapidly and is much longer than either of the bronchi in a 128 

 hour embryo. The epithelium is stratified and, at 80 hours, 

 rests upon a well defined basement membrane. 



