In chick embryos of 4,8,12 and 19 incubation days the ultrastructure of the intrinsic vessels of the optic lobes has been analyzed. During the early periods of the optic tectum vasculogenesis morphological differences have been observed between the vessels which, arising from the external vascular plexus, radially run through the mantle layer, and the vessels of the deep vascular plexus, which precociously arises by branching and anastomosing of the radial trunks immediately beneath the neuroepithelium. Such differences concerning the endothelium thickness, the characteristics of its luminal and abluminal outline, the time of appearance of the endothelial basement lamina and the glial layer, and the width of the perivascular spaces have been related to the growing patterns of the radial vessels which, at least during the first developmental days, are unbranched trunks and precociously differentiate elongating as the mantle layer become more and more thick, and, respectively, of the vessels of the deep plexus, the morphological maturation of which is belated owing to the continuous sprouting of new vascular sprouts.