THE CORPUSCLES OF THE BLOOD 



89 



blood-islands. The spaces between the blood-islands is next exca- 

 vated, some of the angioblasts becoming converted into blood-cor- 

 puscles, others forming the cells of the capillary wall. This process 

 goes on until the circulation is established and capillary formation is 

 complete. The blood-cells thus formed multiply within the vessels. 

 Later on, when the circulation is established, large blood-cells (megalo- 

 blasts), derived from the primitive haemoblasts, also make their 

 appearance in the blood. From the megaloblasts are derived smaller 

 normoblasts, which lose their nuclei and become converted into the 

 erythrocytes similar to those of the adult. 



By some authorities it is held that hsemoblast cells also give rise 

 to lymphoblasts, from which lymphocytes are developed, and possibly 

 leucocytes ; also to myeloblasts, from which myelocytes and leucocytes 

 are developed. All the first blood-corpuscles according to this scheme 

 have a common mesoblastic parent cell, the hsemoblast. 



H^MOBLAST 



I 



Primitive 

 Erythroblast 



Primitive 

 Erythrocyte 



Megaloblast Lymphoblast 



Myelo blast 



Normoblast Lymphocyte Eosinophil Neutrophil Basophil 

 (large and small) myelocyte myelocyte myelocyte 



Erythroblast Leucocyte ? 



| (neutrophil) 



Erythrocyte 



Red marrow 



Lymph 

 glands 



Eosinophil Neutrophil Basophil ' 

 leucocyte leucocyte leucocyte 



. 



Red marrow 



The red corpuscles are formed in mid-fcetal life to a certain extent, 

 and in late foetal life to a large extent, in the liver, spleen, connective 

 tissue, and red bone marrow; but in the last few weeks of foatal life 

 the red bone marrow becomes almost the sole source. The cells arising 

 from the bone marrow are non-nucleated. 



In the human embryo at the fourth week only nucleated cells are 

 present; in the fourth month they form about 25 per cent, of the 

 whole, while at full time but few nucleated corpuscles are found. 



2. After birth the red bone marrow is the sole source of the red 

 blood-corpuscles. Here nucleated erythroblasts are always to be 

 found, derived from megaloblasts and normoblasts. When extensive 

 destruction of the corpuscles is taking place, this erythroblastic tissue 

 shows signs of great activity, and in times of great need the nucleated 

 red cells of the marrow may pass into the blood. 



Generally, however, the erythroblasts multiply by cell division; 

 these cells then exclude or absorb their nucleus, and pass into the 

 circulation as non-nucleated erythrocytes. 



Function. The great function of the red corpuscles is to carry 

 oxygen to the tissues. This it. does by virtue of the haematin portion 

 of its haemoglobin. This function is destroyed on taking blood from 

 an animal, defibrinating it, and heating it to 56 C. On injecting 

 into the animal the blood which has been thus warmed, the corpuscles 



