THE DEVELOPMENT OF THE VASCULAR SYSTEM. 



241 



embryonic area. In the rabbit they begin to develop about the eighth day. A 

 reticulated area appears in the peripheral part of the area opaca and in a short 

 time shows an anastomosing network of channels, in which lie the blood 

 islands. The network then gradually extends toward the embryo and some of 

 the channels converge to form a pair of omphalomesenteric veins and a pair of 

 omphalomesenteric arteries, which behave in the same manner as in the chick 



Int. carotid artery 



Vertebral artery 



^JB Duct of Cuvier 



Cr 



Vitelline vein 

 Vitelline artery. 



Umbilical vein 



Umbilical 



arteries 



Post, cardinal 

 vein 



Aorta 



Post, cardinal vein 



FIG. 216. Reconstruction of a human embryo of 7 mm. Mall. 



Arteries represented in black. A.V., Auditory vesicle; B, bronchus; L, liver; K, anlage of 

 kidney; T, thyreoid gland; III-XII. cranial nerve roots; i, 2, 3, 4, branchial grooves; 

 i, 8, 12, 5 (on spinal nerve roots), ist and 8th cervical, izth dorsal, 5th lumbar spinal 

 nerves respectively. Dotted outlines represent limb buds. 



(Fig. 214). The area vasculosa then gradually enlarges until it embraces the 

 entire yolk sac. 



There are no observations on the early formation of the vascular anlagen in 

 the human embryo, but it is reasonable to suppose they appear in the same 

 manner as in other Mammals. In an embryo in which there is no trace even of 

 a neural plate (Peters' embryo, see p. 90 and Fig. 82), the vascular area 





