979 



TEXT-BOOK OF EMBRYOLOGY. 



Fro. 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 

 (see text). 



The first non-nucleated disks appear in the blood during the second month, 

 and from this time on, the number gradually increases, with a corresponding 

 decrease in the number of nucleated cells. At the time of birth, under normal 



conditions, only non-nucleated red cells are 

 found in the general circulation. 



Jt is a well known fact that red blood cells are 

 constantly disintegrating and disappearing dur- 

 ing the life of the individual; consequently there 

 must be some means of replacing them. The 

 first red blood cells are derived from the ex- 

 traembryonic mesoderm ( ?) ; after this their 

 number increases, to a certain extent at least, 

 by proliferation of the erythroblasts. When the 

 number of erythroblasts is depleted by the 

 transformation of the latter into erythrocytes, 

 the question arises as to the origin of new 

 erythroblasts. The answer to this question is 



not definitely known. As stated before, so long as the formation of new blood 

 vessels continues outside of the embryo in the area vasculosa, so long does 

 the differentiation of erythroblasts continue; but after a time the formation 

 of blood vessels outside of the embryo ceases. By this time the liver has 

 assumed a haematopoietic function, and 

 in it are found vast numbers of erythro- 

 blasts (Fig. 247). It seems possible 

 that these erythroblasts are the direct 

 descendants of those which were formed 

 in the extraembryonic mesoderm, and 

 which are still undergoing proliferation. 

 Later in foetal life the haematopoietic 

 function of the liver diminishes while the 

 ^_ n gradually assumes that function. 

 Toward the end of foetal life the func- 

 tion is taken up by the bone marrow, 

 and during postnatal life, under normal 

 conditions, the marrow alone contains 

 nucleated red blood cells or erythro- 

 blasts (Fig. 250). As to the actual origin 

 of the erythroblasts in the marrow, 

 there is much uncertainty. There is a possibility that they represent the 

 direct descendants of the primary erythroblasts. On the other hand, there is 

 a possibility that the erythroblasts in the bone marrow are being constantly 



FIG. 249. 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. 





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