(1) a transient adrenocortical zone present in some rodents at birth, most notably in mice, situated between the zona reticularis and the suprarenal medulla; it degenerates in males with the secretion at puberty and in females during their first pregnancy; it slowly enlarges in unmated females after puberty and does not degenerate until middle age; the X zone appears to secrete no hormone; SYN: androgenic zone (1). (2) misnomer for the fetal suprarenal cortex of primates. SYN: fetal reticularis (3).