Figure 19.29 A. Sagittal section through the nasal pit and lower rim of the medial nasal prominence of a 6-week embryo. The primitive nasal cavity is separated from the oral cavity by the oronasal membrane. B. Similar section as in A toward the end of the sixth week showing breakdown ofthe oronasal membrane. C. At 7 weeks, neurons inthe nasal epithelium have extended processes that contact the floor of the telencephalon in the region of the developing olfactory bulbs. D. By 9 weeks, definitive oronasal structures have formed, neurons in the nasal epithelium are well differentiated, and secondary neurons from the olfactory bulbs to the brain begin to lengthen. Togther, the olfactory bulbs and tracts ofthe secondary neurons constitute the olfactory nerve (see Fig. 19.30).
Commissures. In the adult, a number of fiber bundles, the commissures, which cross the midline, connect the right and left halves of the hemispheres. The most important fiber bundles make use of the lamina terminalis (Figs. 19.24, 19.27, and 19.30). The first of the crossing bundles to appear is the anterior commissure. It consists of fibers connecting the olfactory bulb and related brain areas of one hemisphere to those of the opposite side (Figs. 19.27 and 19.30).
The second commissure to appear is the hippocampal commissure, or fornix commissure. Its fibers arise in the hippocampus and converge on the lamina terminalis close to the roof plate of the diencephalon. From here the fibers continue, forming an arching system immediately outside the choroid fissure, to the mamillary body and the hypothalamus.
The most important commissure is the corpus callosum. It appears by the 10th week of development and connects the nonolfactory areas of the right and the left cerebral cortex. Initially, it forms a small bundle in the lamina terminalis. As a result of continuous expansion of the neopallium, however, it extends first
anteriorly and then posteriorly, arching over the thin roof of the diencephalon (Fig. 19.30).
In addition to the three commissures developing in the lamina terminalis, three more appear. Two of these, the posterior and habenular commissures, are just below and rostral to the stalk of the pineal gland. The third, the optic chiasma, which appears in the rostral wall of the diencephalon, contains fibers from the medial halves of the retinae (Fig. 19.30).
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