' 



266 THE BLOOD AND THE LYMPH 



spleen, the liver, the bone-marrow, by division of the nucleated corpuscles 

 which are then present in the blood, and perhaps also in the lymphatic 

 tissue. After birth it is probable that most, if not all of the erythrocytes 

 are formed in the red marrow of the bones, especially in that of the skull, 

 the trunk, and the ends of the long bones. After a severe hemorrhage, 

 it is likely that the yellow marrow also gives rise to this sort of blood- 

 corpuscles. The life-period of the erythrocytes has not been definitely 

 determined,, but it may be perhaps some weeks or months. 



It is unlikely that there is any place of destruction of these corpuscles 

 for they probably wear out and gradually disintegrate into the circula- 

 tion. Nevertheless, the liver undoubtedly takes from the worn-out 

 corpuscles the portions containing iron and uses parts of the hemoglobin 

 in the formation of its bile-pigment, bilirubin. The spleen also probably 

 takes some of the iron out of these decomposing corpuscles, and this 

 process may rapidly disintegrate them. 



FIG. 139 

 Centrosome. ^ ^^^ 



^ ^X 



Protoplasmic 

 radiation 



membrane 



Lantanin 

 Chroniatiii 



A leukocyte from the spleen of Proteus. Note the elaborate nucleus and the centrosome. 

 (Siedlocki via Szymonowicz and McCallum.) 



THE LEUKOCYTES or white corpuscles of the blood and lymph^unlike 

 the red plastids, are perfect cells, for they have nuclei as well as cyto- 

 plasm. 



The leukocyte, in general, is a minute more or less globular mass of 

 uncolored granular protoplasm. There is to be found in them all the 

 elements of a cell noted in the first chapter. They have, but in a very 

 restricted degree, the same movements even which we described as 

 characteristic of the ameba. In size leukocytes vary from three to 

 fourteen one-thousandths of a millimeter in diameter. A fair average 

 is ten or twelve microns; they are larger, then, than the red corpuscles. 



It is convenient at the present time to describe four varieties of the 

 leukocytes; small mononuclears (lymphocytes), large mononuclears, 

 polymorphonuclears (myelocytes), and mast-cells. The lymphocytes 



