THE DEVELOPMENT OF THE VASCULAR SYSTEM. 



239 



somewhat eccentric, the chromatin network a little denser and the nucleoli 

 less conspicuous. While these changes are in progress the cells multiply by 

 mitosis. The resulting cells are termed megaloblasts (Fig. 209, a). These 

 continue to multiply by mitosis, the cytoplasm acquiring more haemoglobin 

 and the nuclei becoming more dense, the resulting cells being somewhat 

 smaller and known as normoblasts (Fig. 209, b) . The normoblasts, still divid- 

 ing by mitosis, acquire still more haemoglobin and become erythroblasts (Fig. 

 209, c). These lose their nuclei and thus become erythrocytes, the definitive 

 red blood corpuscles. The manner in which the nuclei are lost is a matter 

 of dispute. Some claim it is absorbed (karyolysis) ; others claim it is extruded 



(karyorrhexis) (Fig. 210); recently the 

 observation has been made that the 

 nucleus with a small amount of sur- 

 rounding cytoplasm escapes from the cell 

 in a manner resembling constriction. 



In the specialization leading to the 

 white blood cell series, the parent stem 

 cell (primitive lymphocyte) proliferates 

 by mitosis and undergoes certain di- 

 vergent changes in its nucleus and cyto- 

 plasm which yield the characters of the 

 various kinds of leucocytes. Some of 

 the cells become polymorphonuclear and 

 acquire neutrophile granules to become 

 neutrophile leucocytes; others acquire 

 acidophile granules as acidophiles; still 

 others, basophile granules as basophiles. 

 The large mononuclear leucocytes, with the transitional forms having the 

 horseshoe-shaped nuclei, possibly represent but slightly modified primitive 

 lymphocytes. The definitive lymphocytes are probably derived from the 

 primitive by division and but slight changes in character. Thus the vari- 

 ous forms of white blood cells would not represent different stages in a series 

 but divergent lines of specialization from a parent stem. 



As mentioned before, the various blood forming organs function as such 

 at successive stages of development of the embryo. The mesenchyme 

 generally, both in the yolk sac and in the body, gives rise to blood cells during 

 the earlier stages and may continue to do so until relatively late in embryonic 

 life as has been demonstrated in the chick. It is interesting to note in this 

 connection that in certain regions endothelial cells may also be transformed 

 into primitive blood cells. In the earlier stages of liver development active 

 haemopoiesis is observed in the sinusoids, probably partly from cells carried in 



FIG. 210. Showing the escape of the nuclei 

 from nucleated red blood cells. Howell. 



I, 2, 3, 4, represent stages of extrusion 

 observed in living cells; a, from circulat- 

 ing blood of adult cat after bleeding four 

 times; b, from young kitten after bleed- 

 ing; c, from 90 mm. cat embryo; others 

 from marrow of adult cat. 



