212 



THE DEVELOPMENT OF THE CHICK 



form from three days sixteen hours to ten days, 

 (three days sixteen hours) the form of the head 

 the brain, eyes, and visceral arches. The cerv 

 strongly marked. There is no neck. The heart 

 protuberance immediately behind the head. The 

 rounded swellings. In Fig. 121 B (five days one 

 vical flexure is less marked; the enlargement of 



In Fig. 121 A 

 is defined by 

 ical flexure is 

 makes a large 

 limb-buds are 

 hour) the cer- 

 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 



