A gland that discharges its secretion into a body cavity (such as the gut) or onto the body surface. Examples are the sebaceous and sweat glands, the mammary glands, and part of the pancreas. Exocrine glands are formed in the embryo from the invagination of epithelial cells. Their secretions pass initially into a cavity (an alveolus or acinus) and then out through a duct or duct network, along which the secretion may become modified by exchange with the blood across the duct epithelium.