STRUCTURE OF FOUR-DAY CHICKS 



135 



Toward the end of the second day of development the om- 

 phalomesenteric arteries establish communication between 

 the dorsal aortae and the vitelline plexus. (See Chap. X and 

 Figs. 29 and 35.) There is now a system of open channels lead- 

 ing from the embryo to the yolk-sac, and back again to the embryo. 

 With the completion of these channels the heart begins to 

 pulsate, circulation of the blood is thereby estabhshed, and the 



Fig. 48. — Diagram to show course of vitelline circulation in chick of about 

 four days. (After Lillie.) For the intra- embryonic vessels see Fig, 47. Abbre- 

 viations; A, dorsal aorta; A.V.V., anterior vitelHne vein; L.V.V., lateral vitelline 

 vein; M.V., marginal vein (sinus terminalis); P.V.V., posterior vitelline vein; 

 V.A., vitelline artery. The direction of blood flow is indicated by arrows. 



blood cells formed in the yolk-sac are for the first time carried 

 into the body of the embryo. 



The course of the vitelline circulation in chicks of four days 

 is shown diagrammatically in Figures 47 and 48. Circulating 



