[Comparative studies of the anatomy of the brain in three species of Agnatha: Petromyzon marinus, Lampetra fluviatilis and Lampetra planeri].

This study has been carried out upon 3 species of Petromyzonts: Petromyzon marinus, Lampetra fluviatilis and Lampetra planeri. According to the values of the index of encephalization (whole brain) and to these of the structures indices (brain subdivisions), similarities as well as differences have been pointed out. Other detail are brought by the relative volume of these subdivisions expressed by a per cent value of the entire brain volume (the ventricular volumes being removed). 1. The indices of encephalization show that the Sea Lamprey is the less encephalized species amongst Vertebrates (with an arbitrary value of 10); but the 2 other lampreys stay at a higher level (respectively 44 and 38) while the Hagfish reaches 54 and the average encephalization index of bony fishes locates at the 100 value. 2. The relative volumes lead to a quantified brain pattern which characterizes the both 3 Lampreys with regard to the other species of Vertebrates: olfactory bulbs are well developed, more than the cerebral hemispheres; at the same time the so-called structure Tegmentum + Medulla oblongata holds about 50% of the entire brain volume. This dual pecularity may be considered as the expression of a primitive status. 3. The structure indices (each of it being fixed at 10 for the Sea Lamprey) are uniformly higher in the 2 Lampetra (with a step of 20 to 30 unities of index). It seems to be, this time, the expression of an inner evolution in Petromyzonts, but remaining the basic pattern of the group. Some examples taken amongst other Vertebrates emphasize this pecularity: Polypterus possess high indices for the cerebral hemispheres (595: processus of telencephalization) and cerebellum (1250). On the other hand the Sturgeon shows low values for all the subdivisions, except cerebellum (549). Finally, the Hagfish withdraws from the brain pattern of Lamprey because its indices are clearly higher for olfactory lobes (141) and diencephalon (113).