Formation of the embryonic blood vessels occurs during the third week of development by two methods: vasculogenesis and angiogenesis. The vasculogenesis entails formation of vessels from blood islands (which are clusters of mesenchyme-derived he-mangioblasts). It results in the establishment of primary vascular beds in the regions where blood vessels are required to be formed. The angiogenesis involves formation of additional vessels by budding and sprouting from existing blood vessels (which have developed as a result of vasculogenesis).