Embryon

Embryon is the term used in several languages, notably French, to denote the early developmental stage of an organism after fertilization. In English, the corresponding term is embryo. Embryology studies how a single fertilized cell becomes a multicellular body with patterned tissues and organs. In humans and many animals, embryogenesis is distinguished from later fetal development and typically encompasses the period from fertilization through the formation of basic organ systems, ending around the end of the eighth week after fertilization.

Animal embryogenesis begins with fertilization and rapid cell divisions (cleavage), forming a morula and then a blastula. The blastula undergoes gastrulation, producing the three germ layers—ectoderm, mesoderm, and endoderm—that establish the body plan and give rise to all tissues. Neurulation forms the neural tube, and organogenesis proceeds as rudimentary organs develop. Throughout these stages, cells transition from totipotent or pluripotent states toward differentiated lineages under genetic programs and signaling cues.

Plant embryogenesis occurs within seeds. A zygote divides to form a mature embryo with apical-basal polarity, a developing shoot and root axis, and embryonic tissues such as cotyledons. Upon germination, the embryo resumes growth as a seedling. Plant embryos show developmental plasticity that supports propagation and seed-based reproduction.

Embryos are central to developmental biology, reproductive medicine, and agriculture. Studies of embryogenesis illuminate congenital anomalies, tissue differentiation, and organ formation. Research involving early embryos raises ethical and regulatory issues, especially concerning embryonic stem cells and assisted reproduction. In conservation and breeding, embryo preservation and seed banking support genetic diversity.