Archive image from page 1058 of Cunningham's Text-book of anatomy (1914). Cunningham's Text-book of anatomy cunninghamstextb00cunn Year: 1914 ( 1st aortic arch Common trunk formed by umbilical and yolk-sac veins Fig. 818. Vena umbilicalis impar Umbilical arteries Vitelline arteries -Schema of Circulation of an Embryo, 1'35 mm. long, with Six Somites. (After Felix, modified.) In the general account of the development of the primitive vascular system and the establish- ment of the fcetal circulation, given in a previous chapter, it was pointed out that the germ of the whole blood-vascular syste
Archive image from page 1058 of Cunningham's Text-book of anatomy (1914). Cunningham's Text-book of anatomy cunninghamstextb00cunn Year: 1914 ( 1st aortic arch Common trunk formed by umbilical and yolk-sac veins Fig. 818. Vena umbilicalis impar Umbilical arteries Vitelline arteries -Schema of Circulation of an Embryo, 1'35 mm. long, with Six Somites. (After Felix, modified.) In the general account of the development of the primitive vascular system and the establish- ment of the fcetal circulation, given in a previous chapter, it was pointed out that the germ of the whole blood-vascular system appears in the wall of the yolk-sac as a series of strands of cells which constitute the angioblast. Some of the DorsaI segmental branches angioblast cells remain in situ and form the blood-vessels of the walls of the yolk-sac and the corpuscular contents of the blood-vessels; other angioblastic cells wander into the embryonic area and form the blood- vessels of the embryo; whilst still others be- come located in the de- veloping liver and other organs, where they be- come foci for the formation of new blood corpuscles. The first blood- vessels developed in the embryonic area are the primitive aorta?. They appear, either just before the embryonic area begins to be folded into the form of the embryo or as the folding is commencing, in the pericardial or anterior region of the embryonic area, where they are continuous with the earlier-formed vessels on the wall of the yolk-sac. From the pericardial region they extend caudalwards, one on each side of the notochord, and as they pass caudalwards they give off a series of dorsal and ventral branches. The dorsal branches are intersegmental in arrangement; they lie in the intervals between the mesodermal somites The ventral branches are more irregular, and are not strictly segmental in arrangement; moreover, they are not only dis- tributed to the wall of the alimentary canal, but they also pass across it to the yolk-sac. Further, tho
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