Individual Development (Ontogenesis)
What is Ontogenesis?
Ontogenesis (also called ontogeny) is the complete developmental history of an individual organism within its own lifetime — from a single fertilized cell (zygote) all the way through growth, maturation, and aging until death. It is distinct from phylogeny, which refers to the evolutionary history of an entire species.
🔑 Three Main Processes of Ontogenesis
📈 Growth
Increase in cell number and body size through cell division (mitosis) and cell enlargement.
🔬 Development
Cell differentiation and specialization — cells become distinct types (muscle, nerve, blood) and form tissues and organs.
⏳ Aging
Gradual decline in cellular function, repair capacity, and physiological efficiency over time.
⚠️ Common Misconception: Many students think ontogenesis only refers to embryonic development. In reality, it spans the entire lifetime — from fertilization through postembryonic growth, maturation, and aging.
📋 Two Major Periods of Ontogenesis
Ontogenesis is divided into two major periods: