[Laminar analysis of neuronal ultrastructure in the midbrain tectum of the steppe turtle].

A layer-to-layer study of the neuronal ultrastructure in the mesencephalic optic centre of T. horsfieldi has been made, the results being compared to those of similar experments with Emys orbicularis L. Neurons of the efferent type (layers IV, V), in contrast to short axon elements (layers, I, 22) are characterized by a greater amount of cytoplasm and by a variety of cytoplasmatic organelles. A few morphological types of neurons are distinguished: light, dark and neurons with a light nucleus and a dark cytoplasm. In two systems of horizontal neurons dark cells predominate to be assumed as being an independent morphofunctional neuronal type. In contrast to E. orbicularis, the neurons of T. horsfieldi show a large amount of axo-somatic synapses on the surface, and a deficiency in spiny outgrowths, both on the apical and basal dendrites.