272 



TEXT-BOOK OF EMBRYOLOGY. 



Fig. 248. — Nucleated red blood 

 cells from marrow of young 

 kitten after bleeding. Howell. 



The upper part of the figure 

 shows mitosis in nucleated red 

 cells; the lower part shows the 

 condensation of the chromatin. 



The nucleus is relatively smaller, round or oval, and contains more distinct 

 chromatin granules and more prominent nucleoli. The cytoplasm contains a 

 trace of haemoglobin and looks homogeneous. Mitoses are frequent. . 



In the further development of the primitive 

 erythroblasts, their nuclei become smaller, the 

 chromatin paler, and the nuc eoli less distinct. 

 The nuclei either disintegrate in the cells or are 

 extruded, the cells thus losing their ability to 

 multiply. The cytoplasm in the meantime in- 

 creases in amount, the outlines becoming slightly 

 irregular, and acquires an abundance of haemo- 

 globin. This type of cell then declines and is 

 ingested by phagocytes derived from the vessel 

 wall, thus perishing without descendant. The 

 primitive erythroblasts and their derivatives 

 constitute but a short lived race of oxygen 

 carriers. 

 The new race of red blood cells, representing the type which lasts through- 

 out the life of the organism, is derived from lymphocytes. Some of the lymph- 

 ocytes, the characters of which have been mentioned in a preceding paragraph, 

 become modified in that the cytoplasm becomes less basophilic, loses its vacu- 

 oles and acquires a trace of haemoglobin; 

 the nuclei become excentric, the chro- 

 matin denser, and the nucleoli less con- 

 spicuous. These modified cells, to which 

 the term megaloblasts is applied, lie in 

 groups all the members of which present 

 about the same stage of differentiation, 

 and measure from 8 to 9 microns n 

 diameter. 



The megaloblasts in turn give rise to 

 other eel's by mitosis, general reduction 

 in size, increase in haemoglobin, increas- 

 ing density of nuclei, and loss of nucleoli. 

 These new cells are called normoblasts 

 (Fig. 246). From these the nuclei are 

 extruded (beginning at the thirteenth or 

 fourteenth day in the rabbit), resulting 

 in the definitive red blood cell or erythrocyte (Fig. 249). 



Thus, in the manner described, red blood cells are being constantly differ- 

 entiated from lymphocytes, which in turn are derived from mesenchyme cells. 



Fig. 249. — Showing the escape of the nuclei 

 from nucleated red blood cells. Howell. 



1, 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. 



