fig 1. T2-weighted fast spin-echo MR image (4000/90/4) of fetal specimen with a gestational age of 11 weeks 5 days shows that the skull base is formed in cartilage before ossification. The basiocciput (straight solid arrow), the postsphenoid (open arrow), containing the pituitary in the cartilaginous sella turcica, and the presphenoid (curved arrow) are seen. Note the optic chiasm (arrowhead). The cerebral hemispheres and anterior third ventricle are derived from the telencephalon. The cerebral hemispheres consist of lateral diverticula that have not yet expanded to cover the brain stem. Note ossification of palate and mandible. fig 2. T2-weighted fast spin-echo MR image (500/90/3) of a fetal specimen with a gestational age of 14 weeks 4 days shows ossification as low signal in the supraoccipital and basioccipital (short straight arrow) centers of the occipital bone. The cartilaginous sella turcica contains the pituitary (open arrow). Note the optic chiasm superior to the chiasmatic sulcus (curved arrow) of the presphenoid. The chiasmatic sulcus is delineated anteriorly by the limbus and posteriorly by the tuberculum sellae (ts)