252 



THE DEVELOPMENT OF THE VASCULAR SYSTEM 



Origin of the Erythrocytes (Red Blood Corpuscles). — These take their origin 

 from the mesamoeboid cells of the embryo and from the premyelocytes of adult 

 connective tissue and bone marrow as erythroblasts. 



i. Erythroblasts (ichthyoid blood-cells of Minot, so-called because they are 



Fig. 243. — Blood-cells from embryos of 12 and 20 mm. X 1160. a, primitive mesamceboid cells; 

 b, ichthyoid cells or erythroblasts; c, sauroid cells; d, sauroid cells; e, cup-shaped nucleated cells; /, 

 erythrocytes, a, b and c are from a 12 mm. human embryo; d, e, and/, from a 20 mm. embryo. 



the typical red blood-cells of fishes), are characterized by the presence of hemo- 

 globin in the homogeneous cytoplasm, which is thus colored red. The nuclei 

 are vesicular with granular chromatin (Fig. 243 b). There is a definite cell 



membrane. The erythroblasts are the only 

 red blood-cells of the first month of em- 

 bryonic development, occurring in embryos 

 of 10 mm. 



2. Normoblasts, termed sauroid blood- 

 cells because they are the red blood-cells of 

 adult reptiles, are first formed in the fiver from 

 the erythroblasts in embryos of the second 

 month, and are predominant at this stage. 

 They are distinguished by their small round 

 nuclei with dense chromatin which stains so 

 heavily that little or no structure can be seen (Fig. 243 c, d). The cytoplasm is 

 larger in amount than in erythroblasts. 



According to Maximow, the primitive erythroblasts in rabbit embryos all degenerate and 

 the normoblasts are developed as a new generation of cells from primitive lymphocytes (mes- 

 amceboids of Minot). 



Fig. 244. — The development of 

 red corpuscles in cat embryos (How- 

 ell), a, successive stages in the de- 

 velopment of a normoblast; b, the 

 extrusion of the nucleus. 



