134 THE STRUCTURE OF THE FOWL 



from the hind - gut on the seventh day and 

 elongate rapidly. 



The liver arises from the entoderm of the 

 duodenum in the form of two diverticula, 

 anterior and posterior, growing outwards as 

 ramifying solid entodermal cords which invade 

 veins. The lumen of the venous cavities is 

 thus converted into numerous cleft-like spaces 

 or sinusoids. The solid cords later become 

 converted into hollow tubes by excavation of 

 their interior, the lumina constituting the 

 bile-channels. 



The pancreas begins to form at about the 

 seventy-second hour. It arises from the duo- 

 denal entoderm as three diverticula — dorsal 

 and right and left ventral — which branch 

 repeatedly without intercommunication. 



On the ventral wall of the pharynx a groove 

 makes its appearance. From this right and 

 left diverticula are developed. The groove 

 becomes the larynx and trachea, and the 

 diverticula develop into the two bronchi and 

 the various air-passages of the lungs. 



Urinary Organs. — The embryonic urinary 

 organs of birds are represented by three 

 structures which appear in succession. 



1. The pronephros or head-kidney develops 



