THE DEVELOPMENT OF THE VASCULAR SYSTEM. 



253 



capsule of the spleen are derived from the original mesenchymal tissue. The 

 blood vessels become dilated in parts of their course to form the cavernous 

 vessels (cavernous veins) which are separated by the pulp cords. The con- 

 nective (reticular) tissue of the pulp cords is a derivative of the mesenchyme, 

 as are also the various types of cells in the cords. The adventitia of the 

 walls of some of the small arteries becomes infiltrated with lymphocytes to 

 form the splenic corpuscles (lymph follicles). 



It is generally recognized that during foetal life the spleen is a hemato- 



Aorta 



Omental 

 bursa 



Right side 



Mesonephros 



Spleen 



Dorsal 



mesogastrium 

 (greater omentum) 



Abdominal cavity 

 (coelom) 



Stomach 

 Left side 



Bile duct Ventral mesoRastrium 

 (lesser omentum) 



FIG. 221. From transverse section through stomach region of a 14 

 pig embryo. Photograph. 



poietic organ, that is, both leucocytes and nucleated red blood cells ai.e pro- 

 duced within it. Normally, the formation of erythrocytes stops at or soon 

 after birth. In severe anaemia or in pernicious anaemia in postnatal life, 

 however, the presence of dividing nucleated red blood cells suggests a return 

 to embryonic conditions. The reticular tissue constitutes the source of these 

 nucleated forms (erythroblasts) . It has also been suggested that the spleen 

 acts as a destroyer of worn-out erythrocytes, for in many cases apparent 

 remnants of the latter have been observed within the cytoplasm of the 



