112 THE BLOOD 



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

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

 same way as in extra-uterine life. 



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

 cleated 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. 





FIG. itc Colored Nucleated Corpuscles, from the Red Marrow of the Guinea-Pig. (E. A. 



Schafer.) 



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. 



The Colorless Corpuscles or Leucocytes. In human blood the 

 white corpuscles, leucocytes, are nearly spherical masses of granular proto- 

 plasm without cell wall. In all cases one or more nuclei exist in each cor- 

 puscle. The corpuscles vary considerably in size but average 10 p, in diameter. 



The number of leucocytes in a cubic millimeter of blood is estimated 

 at 7,500 to 8,000. The proportion of white corpuscles to red, therefore, is 

 about one of the former to 700 of the latter. This proportion is not very 

 constant in health and great variations cccur under the influence of disease, 

 especially in certain infectious diseases in which the number of w r hite corpus- 

 cles is markedly increased. 



After a full meal the white cells in a healthy adult are increased in number 

 about one-third, the increase beginning within an hour, attaining a maxi- 

 mum in three or four hours, and then gradually falling to normal. This 

 process is frequently modified by the character of the food, the greatest 

 increase occurring with an exclusively meat diet, while a purely vegetarian 

 diet has usually no effect. The increase is also more marked in children, 

 and especially in infants. The essential factor is probably the absorption 

 of albuminous matter in considerable quantities. This causes proliferation 

 of leucocytes in the adenoid tissue of the gastro-intestinal tract. 



In pregnancy there is often a moderate increase in the number of white 



