FIG 5. Transitional cortex in the retrosplenial region of the cingulate gyrus. MRM (A) and Nissl stain (B) show progressive thickening of the cortex and increasing definition of the individual layers from the periallocortical regions (BA 29, 30) into the adjacent isocortex (BA 23). BA 29a-c are characterized by a thin cortex with very prominent myelination (asterisk) in a thick layer I. BA 29d shows funnel-shaped broadening of the cortex toward BA 30, with poor lamination, but beginning definition of layer II. BA 30 shows greater cortical thickness and a well-defined layer II. Isocortical BA 23 shows clear definition of layer IV as well as layer II. Sp indicates splenium (magnification x6.25).