Serial paraffin sections (mostly stained with hematoxylin and eosin) of 52 human embryos at stages ranging from 13 to 20 (approximate ovulation age of 5–8 weeks) were examined. The first sign of definitive blood vessels was found to be localized in the apical incisure of the heart of an embryo at stage 14. Blood vessels of this kind closely resembled ‘blood islands’ in appearance, being composed of primitive erythroblasts surrounded by an outer layer of endothelium. At stage 16, a funnel-like invagination of the endothelium was recognized in the posterior wall of the sinus venosus. This structure was considered to represent one of the cardiac veins (probably the middle cardiac vein). A faint endothelial sprout of the left coronary artery was detected at stage 18, while the right one was observed later, at stage 19. Finally, at stage 20, both of the coronary arteries invariably existed with a covering of mesenchymal cells.

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