fig 4. T2-weighted fast spin-echo MR image (4500/87/4) of a fetal specimen with a gestational age of 20 weeks 5 days shows progressive skull base ossification that has extended from the basiocciput (straight arrow) to the postsphenoid (open arrow) to the presphenoid (curved arrow) in the area of the chiasmatic sulcus. fig 5. A and B, Lateral skull radiograph of a fetal specimen with a gestational age of 22 weeks 3 days (A) and T2-weighted fast spin-echo MR image (4000/90/3) of a fetal specimen with a gestational age of 24 weeks 4 days (B) show advancing ossification of the basiocciput (arrow), basisphenoid (postsphenoid, open arrow), and presphenoid (curved arrow). Most of the growth of the head is due to the dominant development of the cerebral hemispheres (O, orbitosphenoid)