282 Vertebrate Embryology 



narrow stalks of the umbilicus and the yolk- 

 sac. The allantois continues to increase in 

 prominence, while the amnion becomes less 

 conspicuous on account of the disappearance 

 of the amniotic fluid. 



By the thirteenth day, the feathers are 

 generally distributed over the body, and their 

 form and color may be seen through the thin 

 walls of the sacs in which they are still en- 

 closed. They remain enclosed in the sacs 

 until the nineteenth day, when they are an 

 inch or more in length. 



On the thirteenth day, nails and scales ap- 

 pear on the toes ; and by the sixteenth day, 

 the nails, scales, and beak are all quite firm 

 and horny. 



By the thirteenth day the cartilaginous 

 skeleton is complete, and numerous centres of 

 ossification have made their appearance. 



By the sixteenth day, the white of the egg 

 has disappeared, and the cleavage of the meso- 

 blast has extended entirely around the yolk. 

 On the nineteenth day, the remains of the yolk 

 are withdrawn bodily into the body-cavity, 

 which was not nearly filled by the loops of the 

 intestine. 



The changes that take place in the circulation 



