THE DEVELOPMENT OF THE VASCULAR SYSTEM. 



27a 



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 hematopoietic function. 



Erythrocytes 



is * 





iM 



f 



# 



;4 



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 



/ \ 



/ Megaloblasts 



/ 



Lymphocytes 



/ \ ! 



/' Megaloblasts Erythrocytes 



etc. 



Normoblasts 



Normoblasts 



\ 



Primitive erythroblasts 



Primitive erythrocytes 

 This type disappears 



Erythrocytes 



