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, eves, 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 
