Chapter CXLI: Section 10: Figure 6 shows a slightly later ovum than Figure 5, seen

from the dorsal side. b.p. is the blastopore. In front of that appears a groove, the neural groove, bordered on either side by a ridge, the neural fold (n.f.). This is seen in section in Figure 7; s.c. is the neural groove; n.f., as before, the neural fold. The neural folds ultimately bend over and meet above, so that s.c. becomes a canal, and is finally separated from the epiblast to form the spinal cord. Below the neural groove a thickening of the dorsal wall of the archenteron appears, and is pinched off to form a longitudinal rod, the precursor of the vertebral column, the notochord, shown in Figure 7 (n.c.), as imperfectly pinched off.