Endocrine system is a system of glands that secrete hormones directly into the bloodstream to regulate various bodily functions. Glands are specialized cells that secrete substances. The endocrine system includes both exocrine and endocrine glands, unicellular and multicellular glands, and liver cells. Goblet cells and APUD cells are examples of specialized cells involved in secretion. The formation of glands involves Mesenchyme cells inducing epithelium to proliferate, invaginate, and differentiate to form ducts and secretory portions. Endocrine glands follow similar steps but do not have duct portions.