244 THE DEVELOPMENT OF THE VASCULAR SYSTEM 



to be amoeboid. During embryonic life, the measmoeboid cells multiply 

 rapidly by mitosis and develop successively in the wall of the yolk sac, in 

 the young blood vessels, and in the liver, lymphoid organs, and red bone 



Besides the mesamceboids 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 believed by Maximow (1906; 1908) to produce all 

 types of blood elements, 



Origin of the Erythrocytes. The red blood corpuscles take their 

 origin as erythroblasts frorn the mesamceboid cells of the embryo, and from 

 the premyelocytes of adult connective tissue and bone marrow. 



v / 



FIG. 250. Blood cells from human embryos of 12 and 20 mm. X 1160. a, Primitive mesa- 

 moeboid cells; b, erythroblasts; c, d, e, normoblasts ; /, erythrocytes. (a-c, from 12 mm., d-f, 

 from 20 mm. embryo.) 



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

 they resemble the typical red blood cells of fishes,) are characterized by 

 the presence of haemoglobin 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 erythroblast is the only red blood cell found. 



2. Normoblasts, also termed sauroid blood cells because they resemble 

 the red blood cells of adult reptiles, are first formed in the liver from the ery- 

 throblasts, 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 or no structure can be seen (Fig. 2 50 c, d) . The 

 cytoplasm is larger in amount than in erythroblasts. 



3. Erythrocytes (red blood corpuscles, erythroplastids) are developed 

 in mammals from normoblasts which lose their nuclei by extrusion (Fig. 



