The two parts of which each gland is composed, are known as the cortex or outer portion (literally the bark) and the medulla or inner portion (literally the core).  No clean-cut boundary sharply delimits the two, as strands and peninsulas of tissue of one portion penetrate the other.  In the history of their development in the species and the individual, and in their chemistry and function, a sharp difference contrasts them.

In the embryo, the cortex is derived from the same patch that gives rise to the sex organs, the ovaries in the female, and the testes in the male, described as the germinal epithelium.  How intimately the two sets of glands are connected is neatly pointed by this fact of a common ancestor.  All vertebrates possess adrenal glands.  In the lowest of the vertebrates, Petromyzon, the two parts are distinct, the cells of the cortex-to-be are situated in the walls of the kidney blood vessels, projecting as peninsulas in the blood stream, the blood sweeping over and past them.  The medulla-to-be consists of cells accompanying the vegetative nerves.  Among reptiles, the two become adjacent for the first time, and among birds one part occupies the meshes of the other.  The size of the cortex varies directly with the sexuality and the pugnacity of the animal.  The charging buffalo, for example, owns a strikingly wide adrenal cortex.  The fleeing rabbit, on the other hand, is conspicuous for a narrow strip of cortex in its adrenal.  Human beings possess a cortex larger than that of any other animal.

No definite chemical substance has as yet been isolated from the cortex.  That remains a problem for the investigator of the future.  But certain observations, especially concerning the relation between the development and behaviour of the so-called secondary sex characteristics, those qualities of skin, hair and fat distribution, physical configuration and mental attitudes, which distinguish the sexes, and the condition of the gland, indicate clearly that an internal secretion will be isolated, and that it will in its activity furnish certain predictable features.

Three different layers of cells, arranged in strings, that interpenetrate to form a network directly bathed by blood, that breaks in upon them from open blood vessels, compose the cortex.  Most remarkable is this method of blood supply for it is exceedingly common among the invertebrates and rare among the vertebrates.

In certain disturbances of these glands, especially when there are tumors, which supply a massive dose of the secretion to the blood presumably, peculiar sex phenomena and general developmental anomalies and irregularities are produced.  If the disease be present in the fetus, taking hold before birth, and so brought into the world with the child, there evolves the condition of pseudo-hermaphroditism.  The individual, if a female, presents to a greater or less extent the external habits and character of the other sex.  So that she is actually taken for a man, although the primary sex organs are ovaries, often not discovered to be such except when examined after an operation or death.  How closely such an occurrence touches upon the problems of sex inversion and perversion comes at once to mind.