366 



COMPARATIVE ANATOMY 



this function is vestigial in the spleen, and bone-marrow has usurped the 

 function. Bone-marrow is well-adapted for erythrocyte formation, since 

 the blood there is relatively static and the carbon dioxide tension is high. 

 Blood-cells, however, whether derived from spleen or bone-marrow, 

 become vascular elements only following migration. Hemoblasts are 

 transformed into erythrocytes in the venous sinuses of the bone marrow. 

 Development of the Vascular System. It is now agreed that the 

 blood and blood vessels are mesenchymatous in origin. The heart, 

 however, with possibly the exception of its endothelial lining, develops 

 from the visceral layer of mesoderm and is therefore of epithelial origin. 



ectoderm 

 yolk granule 





mesoderm 

 blood island 





ectoderm 



central cells of 

 blood island 



peripheral cell 

 of blood island 



somatic 

 mesoderm 

 coelom 



endothelial cell 

 lumen 



yolk 



Fig. 305. — Drawings to show the cellular organization of blood islands at three 

 stages in their differentiation. -4, from blastoderm of 18-hour chick; B, from blastoderm 

 of 24-hour chick; C, from blastoderm of 33-hour chick. (From Patten's "Embryology 

 of the Chick.") 



Development of Blood and Endothelium. Blood makes its first 

 appearance in the form of solid masses of mesenchymatous cells known as 

 blood islands in the visceral layer of the mesoderm adjacent to the endo- 

 derm in the extra-embryonic yolk-sac. These blood-islands consist of cell 

 masses which secondarily unite to form a network of solid cords. In this 

 condition, they constitute the angioblast or source of the primary blood 

 cells. The central cells of the cords are transformed into both red and 

 white blood corpuscles, while the peripheral cells become the endothelial 

 Hning of the blood-vessels. The first blood vessels are, therefore, extra- 

 embryonic, and those within the embryo are secondary. 



