The endocardium and the endothelium of the large arteries. The angiogenesis of the capillary networks throughout the embryo would seem to be a response to their immediate environment. Although the heart and the aortae begin as capillary networks, they soon show individualities that indicate a higher order of specialzation. Their form is controlled by a number of factors, including their response to the rapid accumulation of blood plasma, and the hydrostatic effects consequent on the specialization of the cardiac wall, which seems to be limited normally to a defined area of coelomic mesenchyme. Based on Born reconstructions of No. 836, made by Osborne O. Heard.

[The above text and accompanying figure are taken from O'Rhailly, R., and Müller, F.. Developmental Stages in Human Embryos, Carnegie Institution of Washington, Publication 637, 1987.]

[adapted from figure 13-4]