THE DEVELOPMENT OF THE VASCULAR SYSTEM. 



271 



mesoderm (Fig. 209). The actual origin of these masses is not clear. Do the 

 cells represent differentiated mesodermal cells, or are they derivatives of the 

 entoderm, or of yolk cells (merocytes) ? Whatever their origin, the cells are 

 spherical or polygonal in 'shape and contain distinct nuclei (Fig. 210). The 

 superficial cells of these masses become differentiated into thin, plate-like cells 

 which constitute the endothelium of the primary blood vessels; the more cen- 

 tral cells remain spherical and constitute the erythroblasts (Figs. 211, 246). The 

 erythroblasts are all nucleated and for a time are the only blood cells. Later 

 the leucocytes appear, probably apart from the erythroblasts (see p. 274). 



So long as the formation of new blood vessels continues in the extraembry- 

 onic mesoderm, so long does the differentiation of new erythroblasts occur in the 

 same region. Furthermore, the er- i 



ythroblasts proliferate in the blood 

 vessels by mitotic division. Conse- 

 quently the number increases during 

 the earlier stages not only by differ- 

 entiation from mesodermal ( ?) tissue, 

 but also by active division of those 

 cells already present in the vessels 

 (Fig. 248). This division may occur 

 at any point in the vessels, even in the 

 heart, but after the various organs 

 begin to develop, there are certain 

 places where it occurs more freely, 

 such as the liver, spleen and bone 

 marrow. These organs are called 

 h&matopoietic organs and afford con- 

 ditions which are especially favorable 

 for the proliferation of the early blood cells, since in them the blood current 

 is sluggish (see Fig. 247). 



As already stated, the erythroblasts are the forerunners of the red -blood 



cells and are at first all nucleated. During the latter part of the first month and 



during the second month of development in the human embryo, many of the 



erythroblasts begin to acquire hemoglobin in their cytoplasm. The nuclei then 



become very dense,' deeply staining masses (Fig. 248), and -the haemoglobin 



" icrea'sf s in amount. Finally the nuclei disappear from the cells and the latter 



hen* become non-nucleated disks*.wfrich are carried along in the bloodstream, 



/here they constitute the red blood cells or erythrocyte\ vThe wajjn'yhjch 



tuclei disappear is not definitely known. It is held by some that they disappear 



>y disintegration within the cytoplasm; by others that they are extruded from 



he cytoplasm and undergo degeneration (Fig. 249). 



18 



FIG. 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. 



