THE DEVELOPMENT OF THE VASCULAR SYSTEM 



237 



(yolk sac), (2) the body mesenchyme, including the endothelium of the early 

 blood-vessels, (3) the liver and spleen, (4) bone marrow, and (5) the lymph 

 glands. These various structures are functional at successive periods of 

 development of the embryo, but overlap to a certain extent, the marrow and 

 lymph glands being probably the only foci of origin of blood cells in the adult. 

 In the area opaca blood-cell development is initiated in the formation of 

 the blood islands. Some of the mesenchymal cells become less irregular in 

 shape by retraction of their protoplasmic processes and isolation from the 

 general syncytium. They assume amoeboid properties and the cytoplasm 



FIG. 208. Mesenchyme from a rabbit embryo at the time of beginning blood formation. 



Maximow. 



m, Ordinary mesenchyme cells; m', mesenchyme cell in mitosis; /, primitive Wood cell 



(primitive lymphocyte). 



[uires a distinctly basophilic character (Fig. 208). These then represent 

 primitive blood cells, or hcemoUasts. Maximow has given them the name 

 primitive lymphocytes, or lymphoblasts , regarding them as the common an- 

 cestors of all the blood cells. Clusters of these cells constitute the blood 

 islands which are involved in the development of the primitive blood spaces, 

 the superficial cells being transformed into endothelium (see p. 186) and the 

 central cells remaining as primitive lymphocytes. Other primitive lympho- 

 cytes also differentiate in the mesenchyme outside of the blood spaces, 

 afterward probably entering the vessels by virtue of their amoeboid properties. 



