The endocrine system developed during evolution from single cells to multi-cellular organisms as a means of communication through chemical signals that coordinates multiple organic functions. The investigation of these chemical signals in the early years of the twentieth century gave origin to the science of endocrinology, which refers to the study of a group of specialized secretory organs called endocrine glands that deliver their products, called hormones or first messengers, directly into the interstitial space and enter the circulatory system. Endocrine glands are called internal glands to differentiate from exocrine glands, which deliver their products through ducts into the gastrointestinal tract or outside the body.