Embryology First half of third week of gestation epiblastic cells migrate to form mesoderm mesodermal cells differentaite to arterial and venous vessel on the surface of the embryonic nervous system Seventh gestational week vessels sprout branches penetrate developing brain reach the gray-white interface, either loop back to pial surface or traverse entire neural tube, thus epicerebral transcerebral circn eventually connect arterial and venous systems by around the twelfth weekEmbryology First half of third week of gestation epiblastic cells migrate to form mesoderm mesodermal cells differentaite to arterial and venous vessels on the surface of the embryonic nervous system Seventh gestational week vessels sprout branches & penetrate developing brain reach the gray-white interface, either loop back to pial surface or traverse entire neural tube, thus epicerebral & transcerebral circ'n eventually connect arterial and venous systems by around the twelfth week