THE DEVELOPMENT OF THE VASCULAR SYSTEM. 



271 



Dantschakoff, and to a large extent, Saxer, Ruckert, and Mollier are in agree- 

 ment. The theory in question is that all blood cells have a common stem and 

 that this stem is represented by cells derived from the mesenchyme, the entoder- 

 mal origin of any of the cells being denied. 



Maximow's views are briefly outlined as follows: The sites of blood forma- 

 tion are (i) blood islands in the area opaca and (2) mesenchyme in general, 

 especially the mesenchyme of the liver, marrow and lymphoid organs. In 

 adult life all kinds of blood cells arise in marrow, chiefly lymphocytes in lymphoid 

 organs, and possibly wandering cells in the connective tissues. 



In the formation of the blood islands in the area, opaca, the mesenchymal 

 cells become arranged in strands. The central cells of the strands become 

 free and rounded, the peripheral cells 

 lengthen and form endothelium. Both 

 types are thus mesenchymal deriva- 

 tives. Endothelial cells may free 

 themselves from the vessel wall and 

 join the central cells, or may break 

 away from the outside and join the 

 genera mass of mesenchyme. These 

 observations have led to the conclu- 

 sion that endothelium is not a specific 

 tissue but rather a modification of 

 mesenchyme. 



The central cells of the blood 

 islands, known as primitive blood 

 cells, are spherical in shape, smooth, 

 and are from 10 to 12.5 microns in 

 diameter. They contain large vesic- 

 ular nuclei with very fine chromatin granules; the nucleoli are large and 

 irregular. Mitotic figures are frequently seen. The cytoplasm forms a narrow 

 rim around each nucleus, is finely reticulated and contains vacuoles, attraction 

 spheres and centrioles, but no hsemoglobin. Cells of this character are found 

 in rabbit embryos up to nine and one-half days. 



Following this stage, the primitive blood cells give rise to two types of cells, 

 lymphocytes and primitive erythroblasts. The former constitute a minority of 

 the descendants of the primitive blood cells and resemble the large lymphocytes 

 of the adult. The nucleus is relatively large, slightly indented, and somewhat 

 excentric. The cytoplasm surrounds the nucleus as a rim, is markedly baso- 

 phile, contains vacuoles and an attraction sphere, and shows small pseudopodia. 

 These cells are from 6.5 to 13 microns in diameter. The primitive erythroblasts 

 are sharply circumscribed spheres, about the size of the primitive blood cells. 



18 



FlG. 247. From section of liver of a 27 mm. 

 cat embryo, showing erythroblasts in blood 

 vessels. In the upper right hand corner of 

 the figure is a group of non-nucleated red 

 blood cells (erythrocytes). HowelL 



