134 



EARLY EMBRYOLOGY OF THE CHICK 



pressure from the blood welling into it from capillaries. Corre- 

 lated with the pressure conditions characteristic for it, the walls 

 of a vein have much less elastic and muscle tissue than artery 

 walls, and more non-elastic fibers reinforcing them. 



The Vitelline Circulation.^The earliest indication of blood 

 and blood vessel formation is at the chick's source of food supply. 

 Blood islands appear in the extra-embryonic splanchnopleure 



pharyngeal pouches I -IV 



ant. cardinal v 

 aortic arch IV 



aortic arch I 

 ^disappearing 



ext. carotid a. 



ant. 

 cardinal v. 



post, cardinal v. 

 hind-gut 



ext. iliac artery 

 cloaca 



allantoic vein 

 yolk stalk 

 omph. mes. v. 

 omph. mes. a. 



allantoic vein 

 allantois 



allantoic artery 

 proctodaeum 

 post -anal gut 



Fig. 47. — Schematic diagram to show the location of the more prominent 

 internal organs of the four-day chick. Except for the omphalomesenteric 

 arteries and veins paired structures are represented only on the side toward the 

 observer. 



of the yolk-sac toward the end of the first day of incuba- 

 tion, and rapidly become differentiated to form vascular endo- 

 thelium enclosing central clusters of primitive blood corpuscles 

 (Fig. 25). By extension and anastomosing of neighboring 

 islands a plexus of blood channels is formed in the yolk-sac. 

 Further extension of the vitelhne plexus brings it into communi- 

 cation with the omphalomesenteric veins which have been de- 

 veloped in the embryo as caudal extensions of the heart (Fig. 21). 



