effects of chronic hypoxia on microcirculation of the central nervous system during embryonic development in the attempt to know the factors influencing the vasculogenesis and to verify whether the vessel formation and growth are influenced during different ontogenetic periods by oxygen deficiency the intraneural vascular network has been morphometrically analyzed in chicken embryo optic tectum under conditions of aerogenic hypoxia chicken eggs incubated under routine conditions have been halfpainted with melted wax at the 2nd incubation day id fragments of optic tectum isolated from living embryos at the 8th 14th and 17th id have been fixed and embedded according to the usual em procedures a parallel series of normally developed embryos of 8 14 and 17 incubation days has been likewise prepared on semithin sections from the hypoxic specimens and normal control embryos the area occupied by vessels av the number of vessels nv and the diameter of the radially directed ones dv have been evaluated the preliminary results indicate that hypoxia evokes from the 8th to the 17th id an increment of the av parameter due first to microvessel neoformation and enlargement from the 8th to the 14th id then from the 14th to the 17th id only to a growth of new capillary branches the response of the vascular network to the hypoxic condition is more marked from the 8th to the 14th id ti when in relation to the differentiation and the stratification of the neurons the metabolic requirements of the developing tectum are presumably increased o2 deficiency causes severe developmental disorders of the cyto and mieloarchitecture of the tectum the vascular response can apparently prevent actual damages only when the hypoxic condition lasts for a relatively short time