212 



THE DEVELOPMENT OF THE CHICK 



form from three days sixteen hours to ten days. In Fig. 121 A 

 (three days sixteen hours) the form of the head is defined by 

 the brain, eyes, and visceral arches. The cervical flexure is 

 strongly marked. There is no neck. The heart makes a large 

 protuberance immediately behind the head. The limb-buds are 

 rounded swellings. In Fig. 121 B (five days one hour) the cer- 

 vical flexure is less marked; the enlargement of the mid-brain 



Fig. 122. — Embryo of 7 days' and 7 hours' 

 incubation x 5. (After Keibel and Abra- 

 ham.) 



makes a more pronounced protuberance of the head in this region; 

 the heart has retreated farther back into the thorax, and the 

 neck is thus indicated. The main divisions of the limbs are 

 beginning to appear. In Fig. 122 (seven days seven hours) 

 there are marked changes: The cervical flexure is practically 

 lost. The elevation of the head and retreat of the heart into 

 the thorax have produced a well-marked neck. The upper 



