Ultrastructural features of blood island-like structures (A–C) and angioblastic aggregates (D) in the vitreous of a 6.5 WG embryonic eye. Angioblasts (a), erythroblasts (EB), mesenchymal cells (M), and early lumen formation (L) were present at this age. Blood island-like structures (A–C) were composed of loosely arranged mesenchymal cells with free erythroblasts (A), incompletely enveloped erythroblasts (arrowhead) (B), and erythroblasts completely enveloped by mesenchymal cells (C). Angioblastic aggregates (a) initially formed cords (D) with slit-like lumens (L). There was a paucity of basement membrane material surrounding these blood islands and primitive capillaries. (Scale bars = 2 μm in all).