Olfactory structures in staged human embryos

The olfactory region was investigated in 303 serially sectioned human embryos, 23 of which were controlled by precise graphic reconstructions. The following findings in the embryonic period are new for the human. (1) The nasal plates arise at the neurosomatic junction, as do also the otic placodes. (2) Crest comes from the nasal plates later (stage 13) than the maximum production in the neural folds (stage 10). (3) The crest arises and migrates during a much longer time (at least until the end of the embryonic period) than the neural crest of the head, where origin and migration end at stage 12. (4) Olfactory nerve fibres enter the brain at stage 17, the vomeronasal fibres and those of the nervus terminalis at stages 17 and 18. (5) Fibre connections between the olfactory tubercle and the olfactory bulb, as well as those to the amygdaloid nuclei, forebrain septum, and hippocampus, develop during and after stage 17. (6) Mitral cells appear late in the embryonic period. (7) Localized, although incomplete, lamination of the olfactory bulb is detectable at the embryonic/fetal transition. (8) Tangential migratory streams of neurons, from stage 22 to the early fetal period, proceed from the subventricular zone of the olfactory bulb towards the future claustrum; they remain within the insular region but are separated from the cortical plate. (9) In future cebocephaly morphological indications may be visible as early as stage 13. The various findings are integrated by means of staging, and current information for the fetal period is tabulated from the literature.