Malpighi was the first to announce that all secreting glands are essentially composed of tubes, with blind extremities. Müller, by his laborious researches, has brought this department of the anatomy of glands to its present comparatively perfect condition. Purkinje announced his hypothesis of the secreting function of the nucleated epithelium of the gland ducts, but made no statement to shew that he had verified it by observation. Schwann suggested that the epithelium of the mucous membranes might be the secreting organ of these surfaces. Henle described minutely the epithelium cells which line the ducts of the principal glands and follicles, but did not prove that these are the secreting organs. The same anatomist has stated, that the terminal extremities of certain gland ducts are closed vesicles, within which the secretion is formed, and which contain nucleated cells. Henle has not, therefore, verified the hypothesis of Purkinje, although he is correct in stating that the terminal vesicles of certain gland ducts are closed. It will be shewn, in the course of this paper, that the secretion is not formed, as Henle has asserted, in the closed vesicles, but in the nucleated cells themselves.