Embryology is the study of the formation and development of an embryo and fetus. It is a branch of biology that deals with gamete formation, fertilization, and embryogenesis. Embryology encompasses the growth of both plant and animal embryos, but in a more general sense, it is studied in animals or humans when referring to a medical sense. The study of embryology has been around since the time of the Greek philosopher Aristotle, who debated whether the embryo was a preformed, miniature individual or an undifferentiated form that gradually became specialized. Embryology was based on descriptive and comparative studies before the widespread use of the microscope and the advent of cellular biology in the 19th century. Embryology is central to evolutionary developmental biology ("evo-devo"), which studies the genetic control of the development process, its link to cell signaling, its roles in certain diseases and mutations, and its links to stem cell research. Embryology provides evidence for evolution since the embryonic forms of divergent groups are extremely similar/08%3A_Microbial_Evolution_Phylogeny_and_Diversity/8.01%3A_Origins_of_Life/8.1A%3A_Evidence_of_Evolution).