506 



ANGIOLOGY 





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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 angioblasts 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 



j\ _^^^_ new vessels are formed by sprouts 



from vessels already laid down in 

 the form of capillary plexuses. 



The first rudiment of the heart 

 appears as a pair of tubular 

 vessels which are developed in 

 the splanchnopleure of the peri- 

 cardial area (Fig. 457). These 

 are named the primitive aortse, 

 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 noto- 

 chord under the name of the 

 dorsal aorta. The dorsal aortse 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 primi- 

 tive aortse, 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 aortse 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 



II 



ih. 



FIG. 457. Transverse section through the region of the heart in 

 a rabbit embryo of nine days. X 80. (Kolliker.) /, j. Jugular 

 veins, ao. Aorta, ph. Pharynx, som. Somatopleure. bl. Proamnion. 

 ect. Ectoderm, ent. Entoderm. p. Pericardium, spl. Splanchno- 

 pleure. ah. Outer wall of heart, ih. Endothelial lining of heart. '. 

 Septum between heart tubes. 



1 Anatomical Record, 1917, vol. xiii, p. 199. 



- Anat. Anzeiger, 1899, vol. xv. 



