Open Access

George W. Bartelmez, Anatole S. Dekaban

• The primary subdivisions of the brain (telencephalon, diencephalon, mesencephalon, metencephalon, and myelencephalon) have similar relations and comparable functions in all vertebrates. Accordingly, the landmarksthat define their boundaries can be regarded as reliable for following their development. On the basis of a more complete series of well preserved embryos than has been available hitherto, we present evidence that the subdivisions of the adult brain can be traced back to neural-fold stages in which a series of growth centers can be recognized, differing from one another in form, size, and relations. The possibility of following the constrictions between the various subdivisions throughout development has been doubted by some, notably Hochstetter (1919). At present we are convinced that they can be distinguished if certain criteria are followed. These are: (a) constrictions involve the neural tube as a whole; (b) constrictions do not give rise primarily to any neural centers; (c) constrictions change in relative length and width, and in certain stages they become inconspicuous in models. The anatomical descriptions of progressive stages of development have important practical implications. It is known, for example, that congenital malformations of the central nervous system in man are common and that they are responsible for a substantial portion of fetal wastage as well as infant mortality and morbidity. In certain patients comprehensive clinical studies may indicate the underlying abnormality, such as dysraphism, arhinencephaly, hypoplasia of the cerebellum. or absence of the corpus callosum. In addition, anatomical examination of the affected brains may reveal in detail such abnormalities as lyssencephaly, polymicrogyria, or other cortical dysgeneses. These very complex cerebral malformations can only be understood and unraveled in the light of normal development. An investigation of early development of the brain must necessarily begin with a stage in which the major landmarks of the adult brain can be readily identified.As progressively younger stages are analyzed certain landmarks can no longer be recognized, although others persist at least to the third week of gestation. We believe that the evidence on which this study is based can be followed more satisfactorily in this inverted sequence, and the detailed description is so presented. It is followed by a summary of the sequence of events written in the conventional manner, as far as the eighth week of gestation.