[Encephalization of the marine lamprey, Petromyzon marinus (L.). Quantitative analysis of the principle brain subdivisions].

The measure of both the somatic weight (S) and the brain weight (E) on a sample of 13 adult individuals of Petromyzon marinus (L.) leads for this species to the determination of its brain-body weight coefficient of allometry and to the knowledge of its index of encephalization. The value of the first one is 0.556. Its locates the Sea-Lamprey at the highest level of a scale of decreasing values belonging one after the other to Chondrichthyes (0.551), to Teleost fishes (0.487), to Anurans (0.458), to Reptiles (0.43) and at last to Mammals (0.25). This result revalues our previous hypothesis which gives to this kind of statistic a phylogenetic meaning (RIDET et al. 1977). The index of encephalization has been arbitrarily fixed at the 10 value; it is the lowest of all the other indices previously known and consequently it points the Sea-Lamprey as the less encephalized Vertebrate species. The histological study carried out on the brain allows to the knowledge of the volumes of the main encephalic subdivisions. It leads to various results as: 1. The study the relative volumes (that is to say the volume of each subdivision expressed in percentage of the volume of the whole brain) emphasizes the olfactory bulbs and the Tegmentum + Medulla oblongata which have together the highest values amongst the brain percentages of Petromyzon marinus (and also of Lampetra planeri). This peculiarity has no equivalence in the brain structure of the other studied species of Vertebrates and we consider it as the expression of a primitive brain pattern. 2. The isoponderal indices locate also the Sea-Lamprey at the lowest level for all its encephalic subdivisions, as it was previously the case for the whole brain expressed by the index of encephalization. Such an unanimity seems also to be related to a fundamental brain organization. Comparisons between Petromyzon marinus, Lampetra planeri and Myxine glutinosa corroborate the paraphyletic status of the last one in the Agnathes. The differences pointed also between the two first species are less obvious but justify meanwhile a more detailed study of this group (Petromyzontidae) which shall be given soon.