202 



ANATIDJE. 



of developement in the oldest bird I have met with. The 

 degree next in order, or younger, differs in having the 

 horizontal loop of the trachea confined to one side only of 

 the cavity in the sternum, both sides of which cavity are 

 at this time formed, but the loop of the tube is not yet 

 sufficiently elongated to occupy the whole space ; and the 

 third in order, that on the right of the three representa- 

 tions shown below, from a still younger bird, possesses only 

 the vertical insertion of the fold of the trachea ; yet 

 in this specimen the cavity in the posterior portion of the 

 sternum already exists to a considerable extent. 



These are the peculiarities of structure which belong to 

 the tube and the sternum. The bronchise are very short ; 

 but the flexible part intervening between the bone of di- 

 varication and the bronchial rings is considerable, pro- 

 ducing an effect to be hereafter noticed. This elongated, 

 flexible, and delicate portion being defended on each outer 

 side by a distinct membrane, attached to the whole edge 



