292 THE CHICK. 



very late, and does not open into the rectum until about the 

 fifteenth day. The proctodeum in the chick is very shallow, 

 and gives rise only to the outermost portion of the adult cloaca, 

 and to the actual external opening. 



7. The Lungs. 



The lungs arise, during the third day, as a pair of small 

 hollow outgrowths from the ventral surface of the anterior end of 

 the oesophagus. By lateral constriction, the ventral part of the 

 oesophagus, from which the lungs arise, becomes separated off as 

 a median chamber (Fig. 114, Lg) : this lies ventral to the oeso- 

 phagus, and opens in front into the hinder end of the pharynx ; 

 while from its hinder end the lungs extend backwards as poste- 

 riorly directed outgrowths. 



The lungs, after their first appearance, rapidly increase in 

 size ; they give off secondary diverticula, which branch again 

 and again ; and from the finest branches arborescent outgrowths 

 arise at right angles, which become the ultimate spongy sub- 

 stance of the lungs. 



The air sacs, which are structures very characteristic of birds, 

 appear about the eighth day as thin- walled saccular diverticula 

 from the hinder edges of the lungs ; the abdominal air sacs are 

 in the earlier stages the best developed. 



The trachea (Fig. 116, lr) is formed by elongation of the 

 median laryngeal tube, as the neck lengthens and the lungs 

 gradually shift backwards into the thorax. 



From the mode of development of the lungs, as outgrowths 

 from the alimentary canal, it follows that their lining epithe- 

 lium, including the minutest passages, and that of the air sacs 

 as well, is of hypoblastic origin : the rest of the thickness of the 

 lung walls, including all the blood-vessels, is mesoblastic. 



The lungs contain no air, and are not used for breathing, 

 until immediately before the time of hatching ; when the chick, 

 breaking through the shell membrane into the air chamber 

 at the larger end of the egg (Fig. 101, sv), draws air into its 

 lungs for the first time, and, invigorated by the act, proceeds to 

 peck its way out of the shell. 



8. The Liver. 



The liver arises, about the middle of the third day, as a 

 tubular diverticulum from the posterior end of the fore-gut, in 



