HAEMOPOIESIS 245 



250 /). The nucleus, extruded as several small granules or as a whole 

 (Fig. 251), is ingested by phagocytes. 



Emmel (1914), studying cultures of blood cells from pig embryos, has observed the 

 formation of bodies resembling erythrocytes by a process of cytoplasmic constriction. He 

 suggests that his may be their normal method of development in the embryo. 



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 (Fig. 250 /). Although usually cup-like in 

 preserved material, their normal shape is that of a biconcave .disc (Arey, 

 1917). During the later months of fetal life, the red blood corpuscles 

 are developed in the liver, in the red bone 

 marrow, and probably in the spleen. Ac- 

 cording to the view of Minot, the cells from 

 which they take their origin are mesamce- 

 boids which have lodged in the blood- 

 forming organs and undergo cell division 

 and differentiation there. In the bone , 



FIG. 251. The development of 



marrow these cells are known as premyelocytes. red corpuscles in cat embryos 

 They differentiate into both erythroblasts (Howell). a, Successive stages in 

 and myelocytes', from the former normoblasts the development of a normoblast; 



. o, the extrusion or the nucleus. 



and erythrocytes 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. The white blood cells are divided 

 into non-granular and granular types (Fig. 252). According to the mono- 

 phyletic view, it is assumed that both types are derived from the primitive 

 mesamceboid cells of the embryo. 



I. N on- granular Leucocytes: 



1. Lymphocytes are ordinarily about the size of a red corpuscle but 

 some are twice as large. They constitute from 22 to 25 per cent of the 

 leucocytes in adult blood and are developed in the lymphoid organs of the 

 embryo and adult. The spherical nucleus, containing numerous small 

 masses of chromatin, stains darkly and is surrounded by a narrow zone 

 of clear, faintly basophilic cytoplasm. 



2. Large mononuclear leucocytes are two or three times the size of a red 

 corpuscle. They possess a clear nucleus, usually indented, and considera- 

 ble faintly basophilic cytoplasm. They comprise i to 3 per cent of all 

 leucocytes and are developed from the endothelial or reticular cells of 

 lymph glands (Evans, 1914; Kyes, 1915). 



II. Granular or Polymorphonuclear Leucocytes : 



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



