The brain of the La Plata dolphin, Pontoporia blainvillei, was studied with methods of quantitative morphology. The volumes and the progression indices of the main brain structures were determined and compared with corresponding data of other Cetacea, Insectivora and Primates. In Pontoporia, encephalization and neocorticalization are clearly greater than in primitive ("basal") Insectivora. The indices are in the lower part of the range for simian monkeys. The paleocortex is regressive in accordance with the total reduction of the olfactory bulb and olfactory tract. In contrast to the situation in primates, the septum, schizocortex and archicortex are not progressive in Pontoporia. The striatum and cerebellum are strongly progressive, corresponding to the efficiency and importance of the motor system in the three-dimensional habitat. The diencephalon, mesencephalon and medulla oblongata show considerable progression. Obviously, this is correlated with the extensive development of structures of the acoustic system. The superficial correspondence of the brains of dolphins and primates in relative size and in the degree of gyrencephaly is rather a rough morphological convergence than a sign of functional equivalence. It is coupled to a strongly divergent development of the various functional systems in the two mammalian orders according to their specific evolution.

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The early ontogenesis and topography of the mammalian terminalis system was investigated in 43 microslide series of toothed whale and human embryos and fetuses. In early embryonal stages the development of the nasal pit, the olfacto-terminalis placode, and the olfactory bulb anlage is rather similar in toothed whales and humans. However, toothed whales do not show any trace of the vomeronasalis complex. In early fetal stages the olfactory bulb anlage in toothed whales is reduced and leaves the isolated future terminalis ganglion (ganglia) which contains the greatest number of cells within Mammalia. The ganglion is connected with the nasal mucosa via peripheral fiber bundles and with the telencephalon via central terminalis rootlets. The functional implications of the terminalis system in mammals and its evolution in toothed whales are discussed. Obviously, the autonomic component has been enlarged in the course of perfect adaptation to an aquatic environment.

Serial sections of 13 embryos and fetuses of the harbor porpoise from 10 mm crown-rump length up to 167 mm total length were studied. Unlike the adult animals, ontogenetic stages of 18-27 mm crown-rump length still show a typical mammalian olfactory bulb. The olfactory bulb primordium is penetrated by olfactory nerve fibers, the latter passing through the cribriform plate. However, the olfactory bulb anlage is gradually reduced in later stages, its placodal component being largely uncoupled from the telencephalon. As a ganglionlike structure, the remains of the placodal component stay in contact with the nasal septum and mucosa via thin bundles of nerve fibers. The ganglion and plexus can be traced within the meninges until the adult stage of the porpoise. There is strong evidence that they represent the material of the terminalis system, which cannot be distinguished from the olfactory system in earlier stages. A vomeronasal organ could not be detected in the embryonal and fetal material investigated.

Morphogenesis of the brain in a cetacean species has been investigated by means of reconstructions from serial sections of successive prenatal stages of the harbour porpoise (Phocoena phocoena). Four specimens ranging from 10 to 46 mm crown-rump length (CRL) were selected and three-dimensional reconstructions of the developing brains were obtained with the plate model method. External and internal characteristics, established as criteria for staging embryonic development of primates and rodents, revealed that a common ontogenetic plan regarding the chronological sequence of morphogenetic events exists in mammalian orders as different as primates and odontocetes. Comparison of the 10-mm and 11.5-mm CRL harbour porpoise brains with those in other mammalian embryos of a similar ontogenetic stage (stages 16 and 17) showed a high degree of correspondence in morphological features. This ontogenetic age group therefore might still be considered as a generalized mammalian one. However, during succeeding morphogenesis of the Phocoena brain, qualitative and quantitative divergences from other mammalian groups became manifest, such as those found in the 24-mm CRL specimen (corresponding to mammalian stages 20, 21). Early foetuses of the harbour porpoise (46 and 65 mm CRL) already exhibited a variety of typical odontocete brain features, such as absence of olfactory bulb, thick cochlear nerve, and strong progression of brainstem structures. Morphogenesis of the harbour porpoise brain is discussed from a comparative perspective, incorporating the literature on the development of mammalian brains. Part of this study has been published in abstract form (Buhl and Oelschläger: Acta Anat. (Basel) 120:15-16 (Abstract), '84).