244 



THE DEVELOPMENT OF THE VASCULAR SYSTEM 



The Primitive Blood Cells or Mesamoeboids. — These show large vesicular 

 nuclei surrounded by a small amount of finely granular cytoplasm (Fig. 250 a) . 

 They are without a cell membrane and are assumed to be amoeboid. During 

 embryonic life, the mesamceboid cells multiply rapidly by mitosis and develop 

 successively in the wall of the yolk sac, in the young blood vessels, in the Hver, in 

 the lymphoid organs, and in the red bone marrow. 



Besides the mesamoeboids of extra-embryonic origin, totipotent blood-forming cells 

 appear to rise both from the mesoderm of the embryo and from the mesenchymal cells of adult 

 connective tissue; such cells are beheved by Maximow (Arch. f. mikr. Anat., Bd. 67, 1906; 

 73, 1908) to produce all types of blood elements. 



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

 from the mesamoeboid cells of the em.bryo, and from the premyelocytes of adult 

 connective tissue and bone marrow, as erythroblasts. 



Fig. 250. — Blood cells from embryos of 12 and 20 mm. X 1160. a. Primitive mesamoeboid cells; 

 b, ichthyoid cells or erythroblasts; c, d, sauroid cells; c, cup-shaped nucleated cells; /, erythrocj'tes. 

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



1. Erythroblasts (ichthyoid blood cells of Minot, so-called because they re- 

 semble the typical red blood cells of fishes), are characterizedby the presence of 

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

 nuclei are vesicular, with granular chromatin (Fig. 250 b). There is a definite 

 cell membrane. For the first six weeks of development (12 mm.) the er3^tliro- 

 blast is the only red blood cell found. 



2. Normoblasts, termed sauroid blood cells because they resemble the red 

 blood cells of adult reptiles, are first formed in the liver from the erythroblasts, 

 and are predominant in embryos of two months. They are distinguished by 

 their small, round nuclei with dense chromatin which stains so heavily that little 



