[Ultrastructure of the mesencephalic visual center of the steppe turtle]

A layer-by-layer study of ultrastructural synaptic organization of mesencephalic optic centre (tectum opticum) of T. horsfieldi Gray was made. The predominating type of synaptic contacts is axo-dendritic, however, there are some axo-axonic synapses which are met most frequently in the regions where polysynaptic complexes and glomeneruli (sublayer Ib, sublayer IIa and the upper zone of layer III) are disposed. Axo-somatic contacts are localized virtually in all the neuronic types. In the external layers of the tectum opticum where the terminals of optic afferents are disposed, a considerable morphological variety of nerve terminals is revealed which are distinguished not only by their ultrastructural parameters but also by the character of organization in synaptic complexes at different levels of the cortical plates of the investigated centre.