THE PRIMITIVE BLOOD-CELLS OR MESAMCEBOIDS 253 



3. Erythrocytes (red blood corpuscles, blood plastids) (Minot) are developed 

 in mammals from normoblasts which lose their nuclei by extrusion (Fig. 243 F) . 

 The nucleus may be extruded as several small granules or as a whole (Fig. 244) . 



Emmel (Amer. Jour. Anat., vol. 16) has studied cultures of blood-cells from pig em- 

 bryos and has observed the formation of bodies resembling erythrocytes by the fission of the 

 cytoplasm. He suggests that this may be their normal method of development in the em- 

 bryo. 



The first red blood corpuscles are spherical and are formed during the second 

 month chiefly in the liver. During the third month the enucleated erythrocytes 

 predominate and are disc-like or cup-shaped in form (Fig. 243 /) . During the later 

 months of fetal life, the red blood corpuscles are developed in the liver, in the red 

 bone marrow and possibly in the spleen. According to the view of Minot, the 

 cells from which they take their origin are mesamceboids which have lodged in 

 the blood-forming organs and undergo cell division and differentiation there. In 

 the bone-marrow these cells are known as premyelocytes. They differentiate into 

 both erythroMasts and myelocytes; from the former normoblasts and erythro- 

 cytes arise; from the myelocytes the granular leucocytes are developed. Soon 

 after birth the red bone marrow is the only source of new red blood corpuscles. 



Origin of the Leucocytes, or white blood-cells (Fig. 245). — These are divided 

 into non-granular and granular types. It is assumed that both types are derived 

 from the primitive mesamceboid cells of the embryo. 



I. Non-granular Leucocytes. 



1. Small lymphocytes (22 to 25 per cent, of the leucocytes in adult blood) are 

 regarded as young leucocytes. They vary from 4 to 7.5 m in diameter and are 

 developed in the lymphoid organs of the embryo and adult. The large nuclei 

 containing several connected masses of chromatin stain darkly and are surrounded 

 by a narrow zone of clear basic cytoplasm. 



2. Large mononuclear leucocytes (1 to 3 per cent, of leucocytes) are developed 

 from the endothelial cells lining the medullary sinuses of the lymph glands. This 

 may be demonstrated by intra vitam staining with trypan blue (Evans, Anat. 

 Record, vol. 8, p. 99, 1914). 



II. Granular or Polymorphonuclear Leucocytes. 



The blood-forming cells lodged in the red bone marrow are known as pre- 

 myelocytes. They give rise to myelocytes, cells with round or crescentic nuclei and 

 granular cytoplasm. Similar cells are developed in the lymphoid organs. By 

 undergoing changes (1) in the form and structure of their nuclei, (2) in the size 



