CIRCULATION AND RESPIRATION 



137 



clotting blood is believed to indicate that they play a role in 

 clotting similar to that of the blood platelets of mammalian 

 blood, described below. 



Human Blood Corpuscles. — The erythrocytes of human blood 

 are non-nucleated, biconcave circular disks (Fig. 87). They 

 average about 5,000,000 per cubic millimeter of blood. Nucle- 



Fig. 87. — Group of red corpuscles and two leucocytes (l) as seen in a fresh human 



blood preparation. X 900. 



ated erythrocytes are present in the blood stream of the human 

 embryo but not in later life under normal conditions (Fig. 88). 

 Erythroblasts, the cells which give rise to erythrocytes, are found 

 during fetal life in the liver and the spleen, and in postnatal life, 

 mainly in the red bone marrow. The erythrocyte of the adult 

 is a cell that has lost its nucleus. Though incapable of amoeboid 

 movement, erythrocytes do leave 

 the blood stream under certain 

 conditions, through the capillary 

 walls. The duration of an eryth- 

 rocyte in the blood stream is 

 relatively short. Some determi- 

 nations indicate that in the rabbit 

 the life of a red blood corpuscle is 

 about 8}i days; in the dog about 

 16 days. Their destruction and 

 removal from the blood stream 

 occur in different ways. They 

 may be engulfed by phagocytes of the spleen or hemolymph 

 glands, or they may first break down into small pieces which are 

 taken up by certain leucocytes or by the endothelial cells of the 

 liver. The bile pigments, formed principally by the liver, are 

 derived from the hemoglobin of the erythrocytes. 



The leucocytes are nucleated and lack hemoglobin. The 

 average number in a cubic millimeter of blood is 7,000. They 



Fig. 88. — The development of red 

 corpuscles in cat embryo, a, succes- 

 sive stages in the development of 

 erythroblasts, b, the extrusion of the 

 nucleus. (From Stohr, Textbook of 

 Histology, by Lewis. P. Blakiston's 

 Son and Company. By permission.) 



