THE LYMPHATIC SYSTEM 289 



sinuses, are developed to form the spaces of the splenic pulp. Mall has shown 

 by injecting pig embryos that in the younger fetuses the blood-vessels of the spleen 

 form a closed system. In fetuses of 10 to 12 cm. the capillaries enlarge, giving 

 rise to definite capillary units which drain into the veins through openings in 

 their syncytial endothelium. The enlarged cavernous capillaries form the spaces 

 in the splenic pulp. 



Lifschitz has shown that, in human embryos between 15 and 30 cm. long, red 

 blood-cells are actively formed in the splenic pulp around the giant cells. The 

 lymphoid tissue of the spleen first appears as ellipsoids about the smallest arteries 

 in fetuses of four months. At seven months the ovoid splenic corpuscles appear 

 as lymphoid nodules about the larger arteries. The lymphoid tissue is not 

 formed from tissue of the blood-vessels, but, like the lymph nodules of lymph 

 glands, is developed around them from the mesenchyma. 



