CHAPTER XXXV. 

 DEVELOPMENT OF THE THIRD DAY 





Fig. 287. 

 64-hour chick embryo. (41 somites.) 



It is upon this day that more structures make their first appearance 

 than on any other single day of the chick's entire embryonic life. The 

 blastoderm itself has increased in size so that it covers almost one-half 

 the yolk surface. The white of the egg has decreased in amount so that 

 the vascular area has been brought closer to the surface under the shell, 

 making aeration of the blood easier. 



The sinus terminalis reaches its greatest functional activity during 

 this day, and the vitelline veins have been brought in close contact with 

 the vitelline arteries by the growth of the embryo. 



The blood, which the vitelline or omphalomesenteric arteries bring 

 to the sinus terminalis, still flows headward and tailward as before. The 

 portion flowing toward the head returns to the embryo through two 

 large vessels lying parallel to the long axis of the embryo, but some- 

 times there is only one of these the one emptying into the left vitelline 

 vein. Even if the two vessels are present, the left is the larger. 



It is on the third day also that the single posterior vessel, which 



