S8 



THE STUDY OF CHICK EMBRYOS 



connects with the trunk of a vitelline artery which was developed in the vascular 

 area and conveys the blood to it (Fig. 57). Caudal to the vitelhne arteries the 

 dorsal aortiE rapidly decrease in size and soon end. 



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 



Eind-braiir 



Notocliord 



Ectoderm 



Lens vesicle- 



Cavity of fore-bra ill 



Ant. cardinal vein 



Aorlic arch 1 



Optic vesicle 

 Prosencephalon 



Fig. 58. — ^Transverse section through the fore-brain and eyes of a fifty-hour chick embryo. X 50. 



brain vesicles (Fig. 60). Caudal to the atrium of the heart, two small posterior 

 cardinal veins are developed. They lie in the mesenchyma of the somatopleure 

 laterad in position (Fig. 63). Opposite the sinus venosus the anterior and pos- 

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

 of Cuvier) which open into the dorsal wall of the sinus venosus (Fig. 57). The 

 primitive veins are thus paired like the arteries, and like them develop by the en- 

 largement of channels in a network of capillaries. 



The following series of transverse sections from an embryo of this stage shows 



