DEVELOPMENT OF THE CHICK 147 



before hatching is evidenced by the chirping 

 sounds which may be occasionally heard. 



With the help of the " egg-tooth " the chick 

 makes a small hole in the shell of the egg, and 

 pulmonary respiration is fully established. 

 The initial small opening is rapidly enlarged, 

 and on the twenty-first day the chick emerges 

 from the shell. 



10 a 



