[Comparative volumetric analysis of the principal subdivisions of the telencephalon in saurian reptiles].

The volumetric measure of the main subdivisions of the telencephalon has been carried on 24 species of Lizards and 2 species of Snakes. The studied structures are termed as follows: main and accessory olfactory bulbs, medial cortex (M 1 and M 2), dorsal cortex (D 1, D 2 and D 3), lateral cortex (L), Septum, Tuberculum olfactorium, dorsal and ventral striatum, amygdala and nucleus sphaericus. The analysis of the datas makes use of the SNEL L's formula which relates the volume of the various telencephalic subdivisions (V) to the somatic weight (S): V = k x S alpha. Each alpha value is compared to the value of the coefficient of allometry (A) of the whole brain. The evolutive (phylogenetic) growth of a structure is said fast (or slow) when its corresponding alpha value is higher (or lower) than the encephalic A value. At the cortical level such analysis shows the progressive nature of the dorsal cortex. A partition of the sample into Lacertomorpha (14 species) and Dracomorpha (10 species) (in agreement with the NORTHCUTT'S definition of his Type I and Type II Lizards) corroborates this cortical detail, more distinctly with the second group as well (especially for the D 2 portion). Moreover the high number of progressive structures among the Dracomorpha leads to consider this group as phylogenetically the most advanced in the Order of Lizards. The somatic indices are calculated according the allometric characteristics of the Reference Lizards. The judicious choice of some species allows to show how the development of a biological function may be expressed by the values of the indices of the related structures. For examples: dorsal cortex, dorsal striatum and mode of locomotion; olfactory bulbs, lateral cortex, part M 1 of the medial cortex and olfactory system; D 3 subdivision of the dorsal cortex and visual performances. The duality between Lacertomorpha and Dracomorpha is therefore corroborated by significant differences found for the various indices of a great number of telencephalic subdivisions. It leads moreover to find, grosso modo, two functional types of Lizards: moving-on-the-ground and wellsmelling (mainly Lacertomorpha) on the one hand, arboreal and with a fine vision (mainly Dracomorpha) on the other hand. The isoponderal percentages take an useful illustration of these results; it allows to establish the telencephalic pattern of a standard Lizard in which the pallium keeps the larger part (42%); in the pallium itself, the M 1 subdivision of the medial cortex has the most important percentage, a little more than the D 2 part of the dorsal cortex. A comparative study carried on 2 Snakes gives for Boa constrictor the lowest values of the indices, for almost all the structures. In return Natrix natrix stays, for a great number of structures, close to the level of the legless Lizards; this last result confirms distinctly the two levels of telencephalization already found in Snakes (PLATEL, 1976 a).