68 



THE STUDY OF CHICK EMBRYOS 



of channels in primitive capillary networks between ventral and descending 

 aortae. Opposite the sinus venosus the paired aortic trunks fuse to form the single 

 dorsal aorta which extends as far back as the fifteenth pair of primitive segments. 

 At this point the aortae again separate and opposite the twentieth segments each 

 connects with the trunk of a vitelline artery which was developed in, and conveys 

 the blood to, the vascular area (Fig. 55). Caudal to the vitelline arteries the 

 dorsal aortae rapidly decrease in size and soon end. 



Hind-brain 



Notochord 



Ectoderm 



Lens vesicle 



Cavity of , 

 fore-brain 



Chorion 

 Amvion 



Ant.cardinaJ 

 vein 



Aortic arch I 



Optic vesicle 

 Trosencephalon 



Fig. 56. — Transverse section through the fore-brain and eyes of a fifty-hour chick embryo. X 50. 



As in the previous stage, the blood is conveyed from the vascular area to the 

 heart by the vitelline veins, now two large trunks. In the body of the embryo 

 there have developed two pairs of veins. In the head have appeared the anterior 

 cardinal veins, already of large size and lying lateral to the ventral region of the 

 brain vesicles (Fig. 58). Caudal to the atrium of the heart, two smaller posterior 

 cardinal veins are developed. They lie in the mesenchyma of the soma topleure 

 laterad in position (Fig. 60). Opposite the sinus venosus the anterior and pos- 

 terior cardinal veins of each side unite and form the common cardinal veins (ducts 



