THE BLOOD 



393 



blood arises in the first rudimentary blood-vessels con- 

 nected with the heart. These also, at first, are solid, round 



FIG. 145. Transverse section through the head of an embryonic Chick 

 of 36 hours. Below the medullary tube, the two primitive aortse (pa) are 

 visible in the head-plates (s) on both sides of the notochord. Below the 

 throat (d) can be seen the aortal-end of the heart (ae) ; hh, heart-cavity ; 

 hk, heart membrane ; ks, head-sheath, amnion-fold ; h, horn-plate. (After 

 Remak.) 



FIG. 146. -Transverse section through the heart-region of the same 

 Chick (further back than the former). In the heart-cavity (hK), the heart 

 (/i) is still connected by a heart-mesentery (%) with the intestinal-fibrons- 

 layer (df) of the anterior intestine : d, intestinal-glandular layer ; up, 

 primitive vertebral plates; gb, rudiment of the ear-vesicle in the horn- 

 plate ; hp, first rising of the amnion-fold. (After Remak.) 



cords of cells. They then become hollow, while a fluid 

 separates and gathers in the centre, and single cells detach 

 themselves from the rest and become blood-cells. This is 

 equally true of the arteries, which carry the blood from the 



