14 BLOOD 



and a protein, known as globin. While the composi- 

 tion of hsematin is constant, the globin varies in difierent 

 animals. 



Life-history 



Nucleated red corpuscles circulate in the human embryo 

 as early as the third week. From the eighth week, non- 

 nucleated cells begin to take their place. By the time of 

 birth, nucleated forms have disappeared. 



The corpuscles first appear in the yolk sac, and soon after 

 in the chorion and wherever blood-vessels are being formed. 

 Their development is indeed contemporaneous with that 

 of the blood-vessels, both being derived from the same 

 syncytial masses of mesoderm. From the tenth day, the 

 liver is for some time the seat of formation, while after 

 the sixth week, the same function is performed by the 

 spleen. By birth the seat of formation is transferred to 

 the red bone-marrow, where it remains throughout life. 

 Here all stages of formation can be seen in the cells between 

 the blood sinuses. The activity of the marrow is increased 

 by haemorrhage, diminished by impoverisation of the diet. 

 When the formation of corpuscles is rapid, nucleated forms 

 (erythroblasts) appear in the blood. 



After circulating in the blood for an unknown period, 

 the corpuscles are destroyed by phagocytes, chiefly in the 

 spleen and hsemolymph glands. The hberated haemoglobin 

 is transferred to the liver, where it is decomposed and the 

 haematin formed converted into the bile-pigments, bilirvbin 

 and biliverdin. These are excreted in the bile into the 

 duodenum. They are partly converted into stercobilin, 

 the colouring matter of the faeces, partly reabsorbed and 

 excreted in the urine as urobilin. 



THE LEUCOCYTES 



The leucocytes normally number from 6000-8000 per 

 c.mm. of blood. The number is increased during digestion 

 and in nearly all inflammatory conditions. 



