ORIGIN OF VASCULAR TISSUES. 99 



DEVELOPMENT OF THE BLOOD-VASCULAR TISSUES. 



The Blood- Vessels. The earliest blood-vessels within the embryo 

 are networks of delicate channels within the mesoderm. The large vessels 

 of the trunk arise by consolidation and fusion of the axial portions of the 

 network; the extension of the smaller vessels occurs by the growth and 

 conversion of the angioblastic cells with which the primary blood-tubes are 

 intimately connected. The development of new vessels proceeds from the 

 cells constituting the walls of the preexisting channels. These walls consist 

 of delicate endothelial plates from which pointed sprouts grow into the 

 surrounding tissue. These outgrowths are at first solid, but later become 

 hollowed out by the gradual extension of the lumen of the parent vessel. 



FIG. 135. Developing blood-vessels in embryonal subcutaneous tissue; a, large capillary; b, young 

 capillaries ; c, solid protoplasmic outgrowths forming new vessels. X 300. 



All vessels consist at first of a single layer of endothelial cells. This sim- 

 plicity persists in the capillaries, while the walls of the larger vessels become 

 reinforced by additional layers differentiated from the surrounding mesoder- 

 mic tissue. 



The Erythrocytes. The first, and for a time the only, blood-cells 

 within the embryo are the primary erylhroblasts derived from the meso- 

 dermic elements within the angioblastic areas, the blood-islands. These cells, 

 separated by the colorless plasma which appears between them and in which 

 they henceforth float, undergo mitotic division and produce nucleated ele- 

 ments, the primary erythrocytes, that, in turn, give rise to similar corpuscles. 

 The earliest erythroblasts are relatively large round nucleated cells, whose 

 cytoplasm is faintly granular. Their large nuclei contain networks of chro- 

 matin. For a time the cytoplasm is colorless, but soon becomes tinged with 

 hemoglobin. From the primary blood-forming cells, the hemoblasts, later 

 arise large erythroblasts, the megaloblasts. These are succeeded by smaller 

 nucleated cells, the secondary erythroblasts or normoblasts, which are formed 

 chiefly within the capillaries (and possibly surrounding tissues) of the 

 liver and, probably, to a limited extent within the spleen. The nuclei of the 

 normoblasts are not only of smaller size than those of the primary blood- 

 cells, but denser and much more compact. By mitotic division, the normo- 

 blasts give rise to the young nucleated erythrocytes, which lose their nuclei 



