128 



THE BLOOD 



plasm of the cells and their branched network in which these corpuscles 

 lie then become hollowed out into a system of canals enclosing fluid, in which 

 the red nucleated corpuscles float. The corpuscles at first are from about 

 IQ/J. to i6/z in diameter, mostly spherical, and with granular contents, and 

 a well-marked nucleus. Their nuclei, which are about 5^ in diameter, 

 are central, circular, very little prominent on the surfaces of the corpuscles, 

 and apparently slightly granular. 



The corpuscles then strongly resemble the colorless corpuscles of the 

 fully developed blood but for their color. They are capable of ameboid 

 movement and multiply by division. 



FIG. 112. Part of the Network of Developing Blood Vessels in the Vascular Area of a 

 Guinea-pig, bl, Blood corpuscles becoming free in an enlarged and hollowed-out part of 

 the network; a, process of protoplasm. (E. A. Schafer.) 



When, in the progress of embryonic development, the liver is formed, 

 the multiplication of blood-cells in the whole mass of blood ceases, and new 

 blood-cells are produced by this organ, and also by the spleen. These are 

 at first colorless and nucleated, but afterward acquire the ordinary blood 

 tinge, and resemble very much those of the first set. They also multiply by 

 division. The bone marrow also begins to form red corpuscles, though at 

 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. 



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



