Comparative volumetric analysis of the main subdivisions of the brain in saurian reptiles.

The volumetric measure of the main subdivisions of the brain has been carried out on 32 species of Lizards and 3 species of Snakes. The analysis of the allometrical relations between the volume (the weight) of these subdivisions and the body weight shows firstly that the evolutionary or/and adaptative processes are only located at the mesencephalic and metencephalic levels. A more elaborate study leads meanwhile to the conclusion that anothers brain structures - pallium, basal areas, dorsal thalamus - are also implicated, according to the possibility to group the various species into taxomic units (such as family) or in another way; thus it has been possible to corroborate with quantitative datas the NORTHCUTT'S definition (1972) of the Type I (Lacertomorpha) and Type II (Dracomorpha) lizards; the Dracomorpha show "dynamical" structures - pallium and dorsal thalamus - and remind, in some degree the mammal organization. Each structure in each species can be expressed by the way of a relation index; like the encephalization index, the referrence is given by the 6 Lacertidae of the sample. The analysis of these indices is carried according to the legless condition, tree-dwelling life, vision performance and the various possibilities to group the species (taxonomic units such as family or another else); the comparison of the various indices corroborates the pecularities of the Lacertomorpha on the one hand and of the Dracomorpha on the other hand. The isoponderal percentages are calculated taking into account the allometrical relations and thus are better than the relative volumes commonly used. They lead to results previously expressed by the various relation indices. The comparison between Lizards and Snakes leads to the following conclusions: Snakes are less different among themselves than the Lizards, but the species of modern type (Caenophidia) are more telencephalized; they show a proper brain organization; meanwhile the legless condition is expressed, like in the so-called lizards, by a reduced cerebellar volume.