HAEMOPOIESIS 245 



or no structure can be seen (Fig. 250 c, d). The cytoplasm is larger in amount 

 than in erythroblasts. 



3. Erythrocytes (red blood corpuscles, erythroplastids) are developed in 

 mammals from normoblasts which lose their nuclei by extrusion (Fig. 250/). 

 The nucleus, extruded as several small granules or as a whole (Fig. 251), is ingested 

 by phagocytes. 



Emmel (Amer. Jour. Anat., vol. 16, 1914), studying cultures of blood cells from pig 

 embryos, has observed the formation of bodies resembhng erythrocytes by a process of cyto- 

 plasmic constriction. He suggests that this 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, Science, vol. 44, 

 1916). 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 V_^ a 



spleen. According to the view of Minot, the , ^'^' , 'T ^ eveopmen 

 ^ ° red corpuscles m cat embryos (How- 



cells from which they take their origin are mesa- ell), a, Successive stages in the de- 



v'j i-ii 11 !• .1 11 jr velopment of a normoblast; 6, the 



mceboids which have lodged in the blood-iorm- ^^ . , ^, , ' ' 



° extrusion of the nucleus. 



ing organs and undergo cell division and differ- 

 entiation there. In the bone marrow these cells are known as premyelocytes. 

 They differentiate into both erythroblasts and myelocytes; from the former 

 normoblasts and erythrocytes arise, from the myelocytes the granular leuco- 

 cytes 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. 252). — These are divided 

 into non-granular and granular types. According to the monophyletic view, it 

 is assumed that both types are derived from the primitive mesamoeboid 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 immature leucocytes. They vary from 4 to 7.5 ju 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 sur- 

 rounded by a narrow zone of clear basic cytoplasm. 



