The development of the human body; a manual of human embryology . n completed, but are alwaysdeveloped in that portion of the layer which is most inti-mately associated with the yolk-sac and consequentlybecomes the splanchnic layer. The first indication of thevessels is the appearance in the peripheral portion of themesoderm of cords or minute patches of spherical cells(Fig. 120, A). These increase in size by the division of thecells and by their separation from one another (Fig. 120,B), a--clear fluid appearing in the intervals which separatethem. Soon the cells surrounding each cord arrange
The development of the human body; a manual of human embryology . n completed, but are alwaysdeveloped in that portion of the layer which is most inti-mately associated with the yolk-sac and consequentlybecomes the splanchnic layer. The first indication of thevessels is the appearance in the peripheral portion of themesoderm of cords or minute patches of spherical cells(Fig. 120, A). These increase in size by the division of thecells and by their separation from one another (Fig. 120,B), a--clear fluid appearing in the intervals which separatethem. Soon the cells surrounding each cord arrange 240 THE BLOOD. 241 themselves to form an enclosing wall, and the cords, in-creasing in size, unite together to form a network of ves-sels in which float the spherical cells which may now beknown as erythrocytes. Viewed from the surface at thisstage a portion of the vascular area of the mesodermwould have the appearance shown in Fig. 121, revealing adense network of canals in which, at intervals, are groupsof erythrocytes adherent to the walls, constituting what. Fig. 120.—Transverse Section through the Area Vasculosa ofRabbit Embryos showing the Transformation of MesodermCells into the Vascular , Ectoderm; En, endoderm; Me, mesoderm.—(-van der Stricht.) have been termed the blood-islands, while in the meshes ofthe network unaltered mesoderm cells can be seen, form-ing the so-called substance-islands. At the periphery of the vascular area the vessels ar-range themselves to form a sinus terminalis enclosing theentire area, and the vascularization of the splanchnicmesoderm gradually extends toward the embryo. Reach-ing it, the vessels penetrate the embryonic tissues andeventually come into connection with the heart which has 242 THE DEVELOPMENT OF THE HUMAN BODY. already differentiated and has begun to beat before theconnection with the vessels is made, so that when it ismade, the circulation is at once established. Before,however, the vascularization reaches the embry
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Keywords: ., bookcentury1900, bookdecade1900, booksubjectembryol, bookyear1902
