Embryology is the study of the early prenatal development of an animal.
As a descriptive discipline, it has been largely supplanted in college curricula by developmental biology, a discipline that tends to focus on the cellular, genetic, and molecular events that underlie the development of the embryo.
Nonetheless, knowledge of the morphological changes that are the subject of embryology can make anatomy easier to understand and certainly make understanding of birth defects more logical.Development begins with the fertilization of the egg (ovum) by a spermatozoon to form a zygote (see Chapter 28). The ovum and spermatozoon each contribute half of the nuclear chromosomes to the newly formed zygote. The cells of the zygote undergo division, migration, and differentiation to become successively a morula, a blastula, a gastrula, and then an embryo. Strictly speaking, the period of the embryo ends when the various organs and organ systems are formed. The embryo then becomes a fetus that more or less resembles an adult of the same species. The period of the fetus primarily entails increase in size and functional differentiation of organs. In cattle, the embryo becomes a fetus approximately at the end of the second month of gestation. The fetus becomes a neonate (newborn animal) at parturition (birth).