[Tecto-thalamo-telencephalic visual system of the brain in 13-day-old chick embryos].

Light and electron microscopic studies have been made of the nervous tissue in three parts of the tecto-thalamo-telencephalic visual system--i.e. tectum opticum, nucleus rotundus of thalamus and ectostriatum of telencephalon--of 13-day chick embryos. Neuroblasts and neurones at various stages of differentiation were described together with various types of synaptic and nonsynaptic intercellular contacts in the neuropil of these brain structures. Heterochronous maturation of these parts of the visual system in embryogenesis was noted which reflects the level of their phylogenetic maturity. Being phylogenetically more ancient structures, tectum opticum and nucleus rotundus reveal differentiation earlier than ectostriatum which is phylogenetically younger.