176 THE THIKD DAY. [CHAP. 



section of the alimentary tract immediately in front of 

 the postanal gut is somewhat enlarged, and becomes the 

 inner section of the adult cloaca receiving the urinary 

 and genital ducts. The allantois, to whose develop- 

 ment we shall return directly, opens into it ventrally. 



It is to be noted that the two sections of the cloaca 

 of adult birds have a different origin. The inner section 

 being part of the primitive alimentary tract and lined by 

 hypoblast ; the outer being part of an involution of the 

 outer skin and lined by epiblast. 



The lungs are in their origin essentially buds or 

 processes from the primitive oesophagus. 



At a point immediately behind the region of the 

 visceral clefts the cavity of the alimentary canal be- 

 comes compressed laterally, and at the same time con- 

 stricted in the middle so that its transverse section (Fig. 

 62, 1) is somewhat hourglass-shaped, and shews an upper 

 or dorsal chamber d, joining on to a lower or ventral 

 chamber I by a short narrow neck. 



The hinder end of the lower tube enlarges (Fig. 62, 

 2), and then becomes partially divided into two lobes 

 (Fig. 62, 3). All these parts at first freely communicate, 

 but the two lobes behind, partly by their own growth, 

 and partly by a process of constriction, soon become 

 isolated posteriorly (Fig. 60, lg)\ while in front they 

 open into the lower chamber of the oesophagus. 



By a continuation forwards of the process of con- 

 striction the lower chamber of the oesophagus, carrying 

 with it the two lobes above mentioned, becomes gradu- 

 ally transformed into an independent tube, opening in 

 front by a narrow slit-like aperture into the oesophagus. 

 The single tube in front is the rudiment of the trachea 



