Publisher Summary This chapter describes theories of cytogenesis in the adrenal cortex of normal adult mammals. The chapter discusses the two main theories on cytogenesis in the mammalian adrenal cortex and on the functional significance of its three concentric zones is available: (1) the cell migration or escalatory theory, and (2) the zonal theory. According to transformation field hypothesis, the outermost and the innermost layers should be considered two transformation fields that, on appropriate stimulus (ACTH), could be changed into the actively secreting zona fasciculata. Conversely, under conditions of inactivity (for example, hypophysectomy), the zona fasciculata could partly revert into the outer (zona glomerulosa) and inner (zona reticularis) layers. Therefore, only the zona fasciculata would be able to secrete steroid hormones. The cell migration theory is further examined. This theory postulates that new adrenocortical cells arise in the zona glomerulosa, migrate centripetally into the zona fasciculata, and then degenerate and die in the zona reticularis. In addition, the chapter also examines the zonal theory.