A 22-year-old primigravida had a fetal screening ultrasound study at 18 weeks showing a single large cerebral ventricle and fused thalami. On physical examination at birth at 36 weeks' gestation, the infant is small for gestational age and has multiple anomalies, including postaxial polydactyly of hands and feet, cyclopia, microcephaly, cleft lip and palate, and rocker-bottom feet. The infant dies 1 hour after birth. Which of the following CNS abnormalities best explains these findings?