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An ultrastructural study is performed with a quantitative analysis of some parameters of the synaptic organization of the superficial plexiform layer of the three basic cortical zones (hippocampal, dorsal and pyriform) of the forebrain in two aquatic and two land Chelonia species. On the basis of our own and literature data it is inferred that all the basic elements of the axo-spino synaptic complexes (dendritic spines--active synaptic zones--nerve endings) exhibit both stability, ensuring the structural durability and reliability of the contacts' functioning, on the one hand, and plasticity responsible for homeostasis and adequate transformations, on the other. Normally, functional shifts in the fine structure of synaptic apparatus are presumably too slight to conceal the specific features of the nerve centres' synaptic organization. This made feasible a comparative study of the structure of the axo-spinal complexes in functionally dissimilar zones, which revealed the following three categories of characters: (1) stable, common of all the Chelonia species under study and characterizing a particular level of the integration of nervous processes, and (2) variable, ecologically-specific and (3) species-specific, ensuring adaptation changes within a given level of interneuronal organization. It has been revealed that ecological differences in the synaptic organization of the cortical zones in the aquatic and land Chelonia are most pronounced in the pyriform and hippocampal cortices, i.e. zones of exclusively and predominantly olfactory projections respectively, and are considerably less pronounced in the dorsal cortex, the zone of predominantly visual afferentation.

The morpho-functional characteristics of the optic nerve and tectum opticum were studied electronmicroscopically and electrophysiologically after unilateral enucleation in 4 Chelonia species. Different types of degeneration of the myelinated and unmyelinated optic fibers and their endings in the tectum are revealed. Comparison of the types and times of the degeneration of retinal axons and their terminals with changes of the tectum evoked potentials at different times after enucleation gives ground to conclude that myelinated fibers produce nerve endings degenerating on the dark pattern and unmyelinated, all the remaining endings with the light, neurofilamentous, glycogenous, vesicular and other degeneration patterns. A stratal arrangement of the retino-tectal projections of various type fibers is demonstrated. Asynchrony of the process of degeneration of different groups of fibers and the relationship between the rate of degeneration and the diameter of myelinated fibers, as well as the absence of a relationship between the change pattern and the diameters of myelinated and unmyelinated axons are revealed. In land tortoises and water turtles differences are revealed in the set of major degeneration patterns and their endings also in the patterns of stratal distribution of various categories of the retinal fibers and tectum. A possible relationship is proposed between the above differences and ecological properties of the Chelonia groups under study. Also revealed are species-specific features of some substantial aspects of the morpho-functional organization of the levels of the visual analyzer in Chelonia.

A comparative study was made of cytoarchitectonics, neuronal composition and ultrastructural organization of the three basic cortical zones (hippocampal, dorsal and pyriform) of the forebrain of four members of Chelonia (two aquatic and two land species). The features under study fall into three major groups as follows: (1) common to all the Chelonia under investigation and possibly of the entire order; (2) ecologically specific, i.e. common to animals with similar ecology and different in animals with different ecology; (3) species-specific, i.e. differing in individual species. The common structural features of the forebrain cortical plate in Chelonia underlie the morphological substrate responsible for specific functions in different cortical zones. It is precisely with respect to the common features that the dorsomedial dorsal cortex exhibits a more complex neuronal and synaptic organization as compared with other cortical zones. This part of Chelonia cortex is assumed to be homologous to mammalian neocortex. The more plastic ecologically specific and species specific features appear to provide adaptive specializations within a particular level of the nervous system organization.

The neuronal and synaptic organization of forebrain basic cortical zones in Testudo horsfieldi and Emys orbicularis and their dendritic spines have been studied using Nissle, Golgi and electron microscopic methods. It has been shown by comparison of the results that the two spicies have marked differences in the structural organization of the forebrain cortical zones. The cortical formation in Testudo horsfieldi is different from that of Emys orbicularis in a greater diversity of neuronal types, smaller size of neurons, smaller cell density in each cortical zones, the presence of horizontal dendritic terminals in dorsomedial dorsal cortex, the absence of the large neurons in dorsomedial medial cortex, ect. Moreover, in both species the dorsomedial dorsal cortex in comparison with medial and lateral cortex is characterized by a marked complexity of the structural organization (diverse neuronal composition, the presence of the stellate cells, the highest cell density, the smallest neuronal size). The investigation of spines in the different dendritic levels (proximal, middle and distal) of neurons in three cortical basic zones has been shown that in both species it has been observed the tendency to increasing of the spine density from proximal to distal part of dendrite. at all dendritic levels noninvaginated forms of spines predominated. Invaginated spines were recorded at the proximal and middle levels of dendrites and contain more organelles and inclusions than noninvaginated spines. In Testudo horsfieldi and Emys orbicularis differences of spine thin structure and spine density in the cortical basic zones were revealed. Moreover, in both species the spines of dorsomedial dorsal cortex were more numerous, more variable in shape, more abundant in organelles. It was there that a bush-like distribution of spines was found which in evidently a special form of synaptic organization of cortical neurons. The above features of this cortical zone indicates a higher degree of differentiation, suggesting that the dorsomedial dorsal cortex is phylogenetic youth as compared with hippocampus and lobus pyriformis.

Several methods (light optic, electron microscope and electrophysiological) were used to study the optic nerve and the tectum opticum of water turtles (two species) and of land tortoises (two species). As the result of study of the two levels of the visual analyser in four species of Chelonia three groups of signs of the morpho-functional organization were distinguished: signs common for the Chelonia under study; ecologically specific signs--common for water turtles and species-specific traits which are--different in species of the same ecological group. Specific features of structural-functional organization of the visually directed behaviour in Chelonia at the neuronal populations level were shown to correlate with the ecological features to a greater degree than with the systematic position of the species.

The optic nerve and mesencephalic optic centre (tectum opticum, TO) of the tortoise, Testudo horsfieldi, Gray, in the norm and after the enucleation have been studies using Golgi's method, electron microscopy and the electrophysiological technique. The optic nerve comprises about 400,000 fibres represented by two classes of axons: myelinated (10%) and unmyelinated (90%). The diameters of the former vary within 0.3-3 mum, of the latter within 0.3-1.1 mum. The neurogramms of the optic nerve contain two components corresponding to the two rates of conduction through different groups of fibres (1.3 musec and 0.5 musec). The stratified synaptic organization of the TO has been investigated. The optic terminals are shown to join in complex synaptic formations at certain levels of the upper layers of the TO (Strata I, II and III). The process of degeneration in the optic system of Testudo is drastically stretched in time. It was demonstrated by the electrophysiological control that the TO potentials induced in response to the nerve stimulation were not registered any longer only 6-6.5 months after the enucleation. Certain groups of retinal fibres of the optic nerve and their terminals in the TO are shown to degenerate differently and asynchronously. Earlier unknown types of destructive changes of the terminals (vesicular, neurofilamentous, glycogen and others) are described. It is suggested that the different types of degeneration of the optic fibres and their terminals are the result of possible biochemical heterogeneity of the retinal ganglious cells. The comparison of the terms and types of degeneration of different groups of nervous fibres in the optic nerve and their terminals in the TO has revealed some correlations suggesting that the myelinated optic fibres produce nervous terminals degenerating by the "dark" type, whereas the unmyelinated optic axons may be subdivided into several groups, each producing nervous terminals subjected to a definite type of destruction ("clear", "vesicular", "neurofilamentous" etc.). The localization of certain groups of therminals at certain levels of the cortical plate of the TO confirms the electrophysiological data on the stratified organization of the retino-tectal projections in the TO of lower vertebrates. It has been shown by the comparison of the results obtained with those reported for Emys orbicularis, L. that the two species have marked differences in the morpho-functional characteristics of the retino-tectal system, neuronic composition of the TO, the character of destructive changes of the terminals after the enucleation and the component composition of polysynaptic complexes and glomerules. Probably these differences may come from the peculiarities of the perception and transformation of visual information in the species.