THE BLOOD. 163 



millimetre and the haemoglobinometer gives a reading of 50 per cent., 

 each corpuscle only contains half the normal amount of haemoglobin. 



THE ORIGIN AND FATE OF BED BLOOD COEPUSCLES. 



(1) In early embryonic life red blood corpuscles are formed in 

 areas, known as "blood-islands," lying in the area vasculosa of the 

 blastoderm. The blood-islands lie between the mesoderm and the 

 entoderm, and are said to be derived from the latter. They consist 

 of branched cells which unite to form a syncytium, their nuclei mean- 



*^^ 







o*. *: 





meg in in e' //< >" 



FIG. 53. Red marrow of young rabbit. Magnified 450 diameters. (From 

 Schafer's Essentials of Histology. ) 



, erythrocytes ; e', erythroblasts ; e", a coloured cell undergoing mitotic division ; I, a poly- 

 morphonuclear leucocyte ; in, ordinary myelocytes ; m', myelocytes undergoing mitotic 

 division ; eo, an eosinophile myelocyte ; meg, a giant-cell or megakaryocyte. 



while dividing and each new nucleus becoming surrounded by proto- 

 plasm containing haemoglobin. The coloured cells thus formed are 

 known as erythroblasts, and are the red corpuscles of the embryo. 

 They multiply by division. In later embryonic life similar nucleated 

 coloured cells, or erythroblasts, are found undergoing division in the 

 sinus-like blood-vessels of the liver, and also in the pulp of the spleen. 

 Non-nucleated erythrocy tes v like those developed in post-natal life, are 

 formed in the embryo in connective tissue. The connective-tissue cells 

 become coloured by the formation of haemoglobin, and the coloured 

 protoplasm is subdivided into a number of discs, or erythrocytes, which 



