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Henry Gray (1825–1861).  Anatomy of the Human Body.  1918.
 
formed projects from and is attached to the wall of the vessel. Such a mass is known as a blood island and hemoglobin gradually accumulates within it. Later the cells on the surface round up, giving the mass a mulberry-like appearance. Then the red blood cells break loose and are carried away in the plasma. Such free blood cells continue to divide. The term blood island was originally used for the syncytial masses of angioblasts found in the area vasculosa, but it is probably best to limit the term to the masses within the lumen from which the red blood cells arise as Sabin 1 has done. Blood islands have been seen in the area vasculosa in the omphalomesenteric vein and arteries, and in the dorsal aorta.
  The differentiation of angïoblasts from the mesoderm occurs not only in the area vasculosa but within the embryo and probably most of the larger bloodvessels are developed in situ in this manner. This process of the differentiation of angioblasts from the mesoderm probably ceases in different regions of the embryo at different periods and after its cessation new vessels are formed by sprouts from vessels already laid down in the form of capillary plexuses.


FIG. 457– Transverse section through the region of the heart in a rabbit embryo of nine days. X 80. (Kölliker.) j, j. Jugular veins. ao. Aorta. ph. Pharynx. som. Somatopleure. bl. Proamnion. ect. Ectoderm. ent. Entoderm. p. Pericardium. spl. Splanchnopleure. ah. Outer wall of heart. ih. Endothelial lining of heart. é. Septum between heart tubes. (See enlarged image)

  The first rudiment of the heart appears as a pair of tubular vessels which are developed in the splanchnopleure of the pericardial area (Fig. 457). These are named the primitive aortæ, and a direct continuity is soon established between them and the vessels of the yolk-sac. Each receives anteriorly a vein—the vitelline vein—from the yolk-sac, and is prolonged backward on the lateral aspect of the notochord under the name of the dorsal aorta. The dorsal aortæ give branches to the yolk-sac, and are continued backward through the body-stalk as the umbilical arteries to the villi of the chorion.
  Eternod 2 describes the circulation in an embryo which he estimated to be about thirteen days old (Fig. 458). The rudiment of the heart is situated immediately below the fore-gut and consists of a short stem. It gives off two vessels, the primitive aortæ, which run backward, one on either side of the notochord, and then pass into the body-stalk along which they are carried to the chorion. From the chorionic villi the blood is returned by a pair of umbilical veins which unite in the body-stalk to form a single vessel and subsequently encircle the mouth of the yolk-sac and open into the heart. At the junction of the yolk-sac and body-stalk each vein is joined by a branch from the vascular plexus of the yolk-sac. From his observations it seems that, in the human embryo, the chorionic circulation is established before that on the yolk-sac.
  By the forward growth and flexure of the head the pericardial area and the anterior portions of the primitive aortæ are folded backward on the ventral aspect of the fore-gut, and the original relation of the somatopleure and splanchnopleure layers of the pericardial area is reversed. Each primitive aorta now consists of a ventral and a dorsal part connected anteriorly by an arch (Fig. 459); these three parts are named respectively the anterior ventral aorta, the dorsal aorta, and the first cephalic arch. The vitelline veins which enter the embryo through the
Note 1.  Anatomical Record, 1917, vol. xiii, p. 199. [back]
Note 2.  Anat. Anzeiger, 1899, vol. xv. [back]

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