The human brain at stage 17, including the appearance of the future olfactory bulb and the first amygdaloid nuclei.

Abstract

The growth of the cerebral hemispheres rostrally and caudodorsally brings about a deepening of the interlongitudinal fissure, in which blood capillaries for the future choroid plexus develop. Accumulation of mesenchyme basally presages the formation of the nasal septum. The olfactory bulb and tubercle become outlined. The three main areas of the telencephalon, future archi-, paleo-, and neocortex, can be recognized. The amygdaloid body, which is related to the medial ventricular eminence, contains either one or two nuclei. Nerve fibres from the olfactory tubercle arrive and pass through the amygdaloid area. The first indication of a septal nucleus is recognizable. The lateral ventricular eminence is present but not pronounced. The hemispheric stalk joins the cerebral hemispheres to the ventral thalamus and to the diencephalic part of the medial ventricular eminence. The beginning of the future choroid plexus consists in the formation of blood vessels and necrotic changes in the roof of the telencephalon medium and in rostral growth of the anterior choroid artery. Necrotic processes in the future choroid epithelium are now localized at the periphery of the still multilaminar tissue. The sulcus medius and zona intrathalamica delimit the dorsal from the ventral thalamus. The dimesencephalic borderline passes through the commissural fibres in the roof: the rostral part of the commissure is the posterior commissure, the caudal part, the commissure of the superior colliculi. In the mesencephalon, the red nucleus has a laterorostral position with regard to the nucleus of the oculomotor nerve. The cells of the locus coeruleus are well distinguishable. The gustatory fibres begin to separate from the common afferent tract as the tractus solitarius. Inferior and superior salivatory nuclei are delineated.