_Embryology._--The generally accepted opinion at present is that the
cortical substance is derived from the coelomic epithelium covering
the mesoderm of the upper (cephalic) portion of the Wolffian body, and
corresponds to the nephrostomes of mesonephridial tubules (see URINARY
SYSTEM), while the medullary part grows out from the sympathetic
ganglia and so is probably ectodermal in origin. J. Janosik, however
(_Archiv. f. mikrosk. Anat._ bd. xxii. 1883 and _Sitzungsber. d.
Wiener Akad._, 1885), thinks that the cortical part is derived from
the germ epithelium covering the upper part of the genital ridge. C.
S. Minot (_Human Embryology_, 1897) believes that the original cells
which grow in from the sympathetic disappear later, and that the adult
medullary cells are derived from the cortical.
In the early human embryo the adrenals are larger than the kidneys,
and at birth they are proportionately much larger than in the adult.
(For literature see. _Development of the Human Body_, J.P. McMurrich,
London, 1906; and _Handbuch der Entwickelungslehre_, by O. Hertwig,
Jena.)
_Comparative Anatomy._--Adrenals are unknown in Amphioxus and the
Dipnoi (mud fish). In the Cyclostomata (hags and lampreys) they are
said by some to arise in connexion with the cephalic part of the
pronephros, though other writers deny their presence at all (see W.E.
Collinge and Swale Vincent, _Anat. Anz._ bd. xii., 1896). In the
Elasmobranchs and Holocephali the medullary and cortical parts are
apparently distinct, the former being represented by a series of
organs situated close to the intercostal arteries, while the latter
may be either median or paired, and, as they are placed between the
kidneys, are often spoken of as interrenals. In the Amphibia the
glands are sunk into the surface of the kidney. In reptiles and birds
they are long lobulated bodies lying close to the testis or ovary and
receiving an adrenal portal vein. In the lower mammals they are not as
closely connected with the kidneys as they are in man, and their shape
is usually oval or spherical.