The term area FD of Bonin refers to an area of cerebral cortex defined on the basis of cytoarchitecture in the macaque. Located in the most rostral portion of the frontal lobe it is distinguished from area FC of Bonin by an internal granular layer that is thicker and by an increase in the number and size of "cells of va". The authors regarded area FD as a "vast undifferentiated area" in which "sometimes the cells are smaller, sometimes larger; sometimes the cortex is thicker, sometimes thinner, but theses variations seem inconstant and unimportant." Area FD encompassed multiple areas defined by earlier authors, e.g., area 9 of Walker, area 10 of Walker, area 11 of Walker, area 12 of Walker and part of area 25 of Walker ( Bonin-1947 ).