THE DEVELOPMENT OF THE VASCULAR SYSTEM. 273 



The process goes on not only in the area vasculosa of the embryo but also in the 

 mesenchyme in general in the body, especially in the liver in the young embryo 

 (Fig. 247), and in the bone marrow in later embryonic life and postnatal life 

 (Fig. 248). Organs or tissues in which blood-cell formation occurs are spoken 

 of as performing a hcematopoietic function. 



Erythrocytes 



Megakaryocyte 



/ 



w 



V--. Mp*e Fat space 



X l"-Tr. Megaloblasts 



V"' 



Leucocytes Reticulum 



FIG. 250. Section of red bone marrow from rabbit's femur. 



The descent of lymphocytes and red blood cells may be graphically repre- 

 sented in the following manner: 



Primitive blood cells 

 Derived from mesenchyme 



/ \ 



Lymphocytes Primitive erythroblasts 



/ \ I 



/ Megaloblasts Primitive erythrocytes 



/ This type disappears 



Lymphocytes Normoblasts 



/ \ I 



/ Megaloblasts Erythrocytes 



etc. 



Normoblasts 



Erythrocytes 



