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 aortas (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 ; Tcs, head-sheath, amnion-fold ; h, horn-plate. (After 

 Bemak.) 



Fig. 146. — Transverse section through the heart -region of the same 

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

 (7i) is still connected by a heart-mesentery (hg) with the intestinal-fibrous- 

 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 Bemak.) 



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 



