HEART AND VASCULAR SYSTEM. 39 



end of the single tubular heart bifurcates at an obtuse angle to form two large 

 venous roots (primitive veins), which receive the blood from the vascular area on the 

 yolk-sac when this is developed, and pass it on to the heart. These two primitive 

 veins become the vitelline veins. 



The heart begins to beat very soon after its appearance, even whilst still filled 

 only with a colourless fluid, and before receiving blood from the vascular area. 



Fig. 42. VASCULAR AREA OP THE RABBIT OP 10 DAYS. (v. Beneden andJulin.) 

 The arteries and arterial capillaries are represented red, the venous capillaries and veins blue. 



Afterwards, when receiving and propelling the red blood from that area, and 

 especially after it has become elongated and bent upon itself, it is one of the most 

 prominent objects seen on examining the embryo ; projecting as it does freely into 

 the yet widely open ccelom immediately behind and beneath the cephalic region of 

 the body. 



The first vessels to be developed are formed in mesoblast altogether outside the 

 body and within a circular area (vascular area), which surrounds the developing 

 embryo for a certain distance. The first appearance of red blood occurs in the form 

 of isolated red points (blood-islands of Pander), which are scattered about within 

 this area, and are especially numerous at its circumference, where they form an 



