From Sixth Day to Hatching 283 



during the latter days of incubation have al- 

 ready been described, and need not be men- 

 tioned at this place. 



As early as the sixth day, movements of the 

 chick may sometimes be noticed, on opening 

 the egg, but whether they are purely volun- 

 tary, or caused by the effects of the air on the 

 opened egg, it is difficult to say. Soon after 

 this time undoubted voluntary motions do 

 occur : and on the fourteenth day, the embryo 

 shifts its position, so that, instead of lying 

 transversely to the long axis of the egg, it 

 comes to lie with its head towards the large 

 end of the egg, which brings its beak close 

 to the now much-enlarged air-space that was 

 mentioned in the description of the unincu- 

 bated egg (Fig. 33, a). About the twentieth 

 day, the beak is thrust into the air-space, and 

 the chick, for the first time, begins to breathe 

 by means of its lungs. As the pulmonary 

 circulation begins, the blood ceases to flow 

 into the umbilical vessels, and the allantois, in 

 consequence, shrivels up, and is left inside the 

 shell as the chick pecks its way out of the egg. 



