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 aortne (]ja) 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 ; 

 M-, heart membrane; Ics, head-sheath, amnion-fold; /«, 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 (lih), the heart 

 (h) is still connected by a heart-mesentery (hg) with the intestinal-iibrous- 

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

 primitive vertebral plates; gh, 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, Avhile 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 



