THE DEVELOPMENT OF THE VASCULAR SYSTEM 



271 



by the blood stream and partly from primitive blood cells derived from the 

 neighboring mesenchyme (Fig. 249). This function ceases in the liver in later 

 embryonic life. The formation of blood cells takes place in the developing 

 spleen but erythrocyte formation ceases after birth, although following severe 

 haemorrhage the function may be resumed even in adult life. The formation 

 of lymphocytes, however, goes on throughout life in the splenic corpuscles. 



FIG. 249. From the liver of a rabbit embryo, showing formation of red blood cells. Maximow. 

 a, Megaloblasts; a', megaloblast in mitosis; b, normoblasts; c- erythroblasts; en, en', en", endothe- 

 lial cells; h, liver cells; I, primitive lymphocytes; /', primitive lymphocyte in mitosis; 

 , nucleus being extruded from small erythroblast. 



The lymph glands are constant sources of lymphocytes, the parent cells being 

 the large mononuclear cells found in the germinal centers. These cells are 

 regarded as closely allied to the primitive lymphocytes, perhaps even 

 identical, although here in this particular environment giving rise only to the 

 lymphocyte line. 



The bone marrow is an important source of blood cells in the embryo, 

 and in the adult under normal conditions is regarded as the only source of 



