THE VASCULAR SYSTEM 367 



The differentiation of blood corpuscles is correlated with the appear- 

 ance of an intercellular fluid plasma. Thus the blood becomes a Hquid 

 tissue. Connexions of the vitelline vessels are soon established with those 

 formed within the embryo and similar vessels extend into the body-stalk 

 and chorion. Blood is already present in a human embryo of three to 

 four weeks. The extension of the angioblast and the formation of blood 

 channels is believed to occur, not by the sprouting and elongation of the 

 primary trunks, but by the confluence of separate cords of cells, as a 

 drain is built of separate tiles placed end to end. Some blood vessels, 

 however, appear to be formed out of capillary networks by the enlarge- 

 ment of a single channel and the disappearance of the others. Primarily, 

 the walls of the blood vessels consist of a single layer of endothelium. 

 The muscular and connective tissue layers are added from the surrounding 

 mesenchyme. 



There is a divergence of opinion as to the origin of the various types of 

 blood cells of the adult blood. According to the monophyletic theory, 

 both leucocytes and erythrocytes come from a primitive hemoblastic 

 cell, and thus have a common origin. According to the alternative 

 poljrphyletic theory, red and white corpuscles have a diverse origin. 

 Difference of opinion has also arisen as to whether or not the blood 

 corpuscles of the adult are the direct descendants of the primary blood 

 cells of the embryo which have migrated to the various hemopoietic 

 organs and centers of proliferation. The evidence on the whole favors 

 the conclusion that the blood cells of the adult have diverse sources of 

 origin (red marrow, endothelium, lymph nodes, spleen, etc.) and are 

 not the direct descendants of the primitive angioblast cells. 



In the embryo, blood cells arise successively from diverse centers, 

 including the blood islands, vitelline blood vessels, the adenoids, liver, 

 spleen, and finally from bone-marrow. Proliferation of blood cells 

 continues throughout life in the bone-marrow, lymph glands and spleen, 

 but ceases early in other organs. It has been calculated that erythrocytes 

 are manufactured in the adult body at the rate of ten to thirty thousand 

 every second. 



The primary type of blood cell is the hemoblast cell, which contains a 

 relatively large vesicular nucleus and relatively small surrounding cyto- 

 plasm. During its histogenesis, the nucleus of the hemoblast shrinks, 

 while the cytoplasm increases in volume. In this way, the hemoblast is 

 converted into a normoblast, if the cell is to become an erythrocyte. 

 The nucleus of the normoblast, in correlation with the concentration of its 

 chromatin, stains intensely. Eventually, the nucleus of the erythrocyte 

 is extruded bodily, though whether by active migration from the cyto- 

 plasm or by being abandoned by the cytoplasm is uncertain. As a 

 consequence of its loss of a nucleus, the life of an erythrocyte is relatively 



