Vascularization and basement membranes of the chick brain in light microscopy

Blood vessels were first observed in the hindbrain of chick embryos at stage 20 (three to three and one half days incubation). At this time the central nervous system had a complete PAS‐positive basement membrane. The surrounding blood vessels did not have a similar investment. An attenuated basement membrane, continuous with the one on the nervous tissue, enveloped each invading vessel. Invading vascular sprouts entered as solid cords which soon developed lumens and became functional capillaries.