GENEKATION AND DEVELOPMENT. 



205 



seen on the second day, or about twenty-four hours earlier 

 than Aristotle states. About the end of the second day, the 

 rudimentary heart with the vitelline blood-vessels extending 

 over the yolk sac are visible, and, on the third day, they 



FIG. 9. 



EGG OPENED AFTER EIGHT DAYS INCUBATION. 



- ALLANTOIC BLOOD-VESSELS. 

 VITELLINE BLOOD-VESSELS. 



FIG. 10. 



CHICK REMOVED FROM EGG AFTER 

 TEN DAYS INCUBATION. 



become conspicuous. By this time the embryo is raised 

 from the underlying substance of the yolk, being connected 

 therewith merely by a short stalk. During the fourth day, 

 the enclosure of the embryo by the coalescence of the head 

 and tail folds of the amnion becomes complete. On the fifth 



