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In a study of early neuroblast migration in the mesencephalon and rhomb- encephalon (VAAGE, 1965) it was observed that columns of neuroblasts labelled with tritiated thymidine migrated extensively in the mantle zone. In order to determine the route of migration of the neuroblasts, it was essential to define points to which the columns of neuroblasts could be referred. During the investiga- tion a cellfree zone was observed between the mesencephalic and rhombencephalic mantles, presumably identical with the m -segment described by PALMGREN 2 (1921). Several recent investigators (BENGMARK et al., 1953; BERGQUIST and KALLEN, 1954), however, advocate that no morphological boundary exists between the mesencephalon and rhombencephalon. These divergent observations and interpretations of the morphology of the neural tube in the mesencephalon and rhombencephalon prompted me to review the relevant old and recent literature concerning the morphogenesis of the neural tube. It turned out that rather divergent views on this subject are found in the literature. It was therefore deemed to be of interest to undertake a reinvestigation of the early neurogenesis. The results of this investigation are presented in this paper. As will be seen the findings concerning the neurogenesis in the chick support the opinions of several previous investigators, but in addition some new information has been obtained. Since the early neurogenesis is principally identical in chick and man the present investigation, therefore, may shed some light on the early morphogenesis in the central nervous system also in man. I. Introduction A.
