THE BLOOD 



393 



blood arises in the first rudimentary blood-vessels con- 

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



Fig. 145. — Transverse section through the head of an embryonic Chick 

 of 36 hours. Below the medullary tube, the two primitive aort;© (pa) are 

 visible in the head-plates (.9) on both sides of the notochord. Below the 

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

 hk, heart membrane ; Ics, head-sheath, amnion-fold ; li, horn-plate. (After 

 Eemak.) 



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

 Chick (further back than the former). In the heart-cavity (7z7i), the heart 

 (li) is still connected by a heart-mesentery (Ti;/) with the intestinal-fibrous- 

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

 pi'imitive vertebral plates; gh, rudiment of the ear-vesicle in the hoi-n- 

 plate ; Tip, 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 



