446 EMBRYOLOGY OF THE CHICK. 



in size. About the eleventh day one can tell to ^yhat group the 

 embryo belongs, generic characters being oftun plainly marked. 

 Feathers begin to appear as papillfe on the ninth day along the 

 back and rump. These feathers remain in sacs until the nine- 

 teenth and twentieth days, but their colour shines through the 

 sac. A chalky knob appears on the nose on the ninth day, 

 and by the twelfth this has become converted into a soft beak. 

 The cartilaginous skeleton is fully formed by the thirteenth 

 day, so are most of the important muscles ; and on the twelfth 

 day many areas of ossification may be seen in the limbs, ribs, 

 head, &c. 



The loops of the intestines become confined in the abdomen on 

 the eleventh day. The body still is connected by the narrow 

 somatic umbilicus, the allantoic stalk, and suspensory cord of 

 the yolk-sac with the accessory appendages. The whole embryo 

 is now surrounded by the allantois, which is closely applied to 

 the serous membrane against the shell. About the sixteenth day 

 all the white has gone, and the mesoblast is cleaved right round 

 the yolk. On the nineteenth day the yolk-sac is withdra'ivn 

 through the somatic stalk into the abdominal cavity. The 

 embryo is now surrounded by the vascular allantois, the serous 

 membrane, and the amnion. The amnion is continuous at the 

 umbilicus with the walls of the embryo, whilst the serous mem- 

 brane is entirely separated from it. The allantois is continuous 

 with the cloaca, by means of its stalk passing through the 

 umbilicus. 



On the fourteenth day the chick lies with its beak touching 

 the serous membrane and shell membrane, where they form 

 the inner wall of the air-chamber. On the twentieth day the 

 chick pushes its beak into this chamber and breathes air. At 

 this time the pulmonary circulation comes into play, and the flow 

 up the umbilical vessel ceases. The allantois shrivels up, the 

 umbilicus closes completely, and the cliick pecks its way out of 

 the shell, leaving behind the allantois, amnion, and serous 

 membrane. 



