[Period of degeneration and features of the laminar distribution of the visual nerve endings in the midbrain tectum of the steppe turtle (Testudo horsfieldi, Gray)].

A study of ultrastructural changes in the neuronal terminals of optic fibres at the level of tectum opticum (TO) in different periods after unilateral enucleation has revealed the process of degeneration in the optic system of Testudo horsfieldii as well as in Emysorbicularis L. to take a considerable time: from two weeks to six months. The character of destructive changes and the mode of utilization of desintegrated elements are dissimilar in different groups of retinal axons in the optic nerve and their terminals in TO. A comparison of the degeneration periods and the character of the changes in the optic fibres has made it possible to establish the following approximate correlations: all the nerve terminals of myelinated fibres degenerate after the "dark" type, whereas the terminals of non-myelinated axons are subjected to other kinds of transformations ("light", vesicular", neurofilamentous", "glycogen"). The analysis of the distribution of degenerating nerve terminals in the TO layers has demonstrated that there is a regularity in localization of different nerve terminals at certain levels of TO which is indicative of the layered organization of retino-tectile connections. Considerable differences in the neuronal and synaptic organization of TO have been revealed as well as in the character and the time of destructive changes of retino-tectile fibres after enucleation in T. horsfieldii and E. orbicularis.