130 THE BLOOD 



first in small amounts only. This function develops rapidly, however, so 

 that at birth the marrow represents the chief seat of production of the red 

 cells. Nevertheless, nucleated red cells are usually found at birth, sometimes 

 in considerable quantities in the liver and in the spleen. Non-nucleated 

 red cells begin to appear soon after the first month of fetal life, and gradually 

 increase so that at the fourth month they form one-fourth of the whole amount 

 of colored corpuscles. At the end of fetal life they almost completely re- 

 place the nucleated cells. In late fetal life the red cells are formed in almost 

 the same way as in extra-uterine life. 

















FIG. 113. FIG. 114. 



FIG. 113. Multiplication of the Nucleated Red Corpuscles. Marrow of young 

 kitten after bleeding, showing above karyokinetic division of erythroblast, and below the 

 formation of mature from immature erythrocytes. (Howell.) 



FIG. 114. Shows the Way in which the Nucleus Escapes from the Nucleated Red 

 Corpuscles, i, 2, 3, 4, represent different stages of the extrusion noticed upon the living 

 corpuscles, a, Specimen from the circulating blood of an adult cat, bled four times; b, 

 specimen from the circulating blood of a kitten forty days old, bled twice; c, specimens 

 from the blood of a fetal cat, 9 cm. long. Others from the marrow of an adult cat, two of 

 the figures showing the granules present in the corpuscles, which have been interpreted 

 erroneously as a sign of the disintegration of the nucleus. (Howell.) 



Various theories have prevailed as to the mode of origin of the non- 

 nucleated colored corpuscles. For a time it was thought that they were of 

 endoglobular origin, and merely fragments of some original cell, being pro- 

 duced by subdivision of the cell body itself. This theory easily accounted 

 for the absence of the nuclei, but it has not been supported by recent investi- 

 gations. At present it is the general belief that the non-nucleated cells, or 

 erythrocytes, are derived from nucleated cells by a process of mitotic division, 

 and further that their nuclei gradually shrink or fade and are then extruded. 

 The use of some of the more recent stains seems to prove that there are traces 

 of nuclear material in the non-nucleated corpuscles. 



After infancy and early childhood the origin of erythrocytes is practically 

 limited to the red marrow of the bones. The mother cells, or erythroblasts, 

 are constantly forming and setting free erythrocytes, the rate varying greatly 

 at different periods. 



