SECOND HALF OF SECOND DAY 483 



extend laterad to meet the vessels which develop in the vitelline plexus 

 outside the embryo, and which extend inward toward the embryo. The 

 omphalomesenteric veins (those lying within the embryo) eventually 

 become one with the vitelline veins (those lying in the extra-embryonic 

 area) and thus establish the afferent vessels of the vitelline circulation. 



The efferent vessels develop at about forty hours. They have a dual 

 origin. The embryonic vessels consist of the branches of the dorsal 

 aortae which extend outward where they meet with the extra-embryonic 

 arteries growing toward the embryo to meet with, and become confluent 

 with, the embryonic efferent vessels, now being known as the 

 omphalomesenteric arteries. 



It is at about the thirty-second hour that the heart begins to con- 

 tract irregularly, although the maximum rate (150 to 180 per minute), 

 is not reached until after 100 hours of incubation. 



A portion of a. cross section of a 54 hour chick embryo 



through the solid anlage of the pronephric tubules in the 



region of the beginning of the Wolffian duct. The nephros- 



tomes are just beginning to form, neph.st., nephrostome ; 



u.n. pronephric ducts; w, Wolffian duct. (After Kolliker.) 



THE PATH OF A BLOOD-CORPUSCLE 



It is well to follow a corpuscle through its entire circulation at this 

 time. With the contraction of the heart, the corpuscle will be sent 

 through the ventral aortae, along the dorsal aortae, out through the 

 omphalomesenteric arteries to the plexus of vessels on the yolk. 



It will be remembered that there are various membranes surround- 

 ing the yolk. These contain many small vessels which absorb the yolk. 

 As there must be an oxygenation of the blood, this vitelline circulation 

 must also assist in this function until the allantois, shortly to be de- 

 scribed, is formed. This aeration can be accomplished on account of the 

 great area these membranes cover, which permits a wide field from 



