Ontogeny, also known as morphogenesis, explores the lifespan of an individual organism, especially its growth from fertilization to maturity. Ontogeny is closely related to developmental biology, and all the sciences which revolve around the development of organisms. This includes psychology and cognitive neuroscience. As such, ontogeny is intensely useful both for modern medicine and the study and classification of animals.

The development of different animals can vary in the extreme. For example, humans, and many other mammals, start as a fertilized egg, and pass through several stages of growth, from embryo to fetus to infant, within the womb. Reptiles and birds generally lay fertilized eggs, instead of nurturing them in a womb, and the embryo feeds on the protein within the egg. Ontogeny also studies an organism's journey from infancy to sexual maturity, such as a caterpillar forming a cocoon, undergoing metamorphosis, and emerging as a butterfly. Ontogeny also takes allometry into account. Allometry is the specific branch of study focusing on the details of proportional growth from a child to an adult. This includes the shifts in bone structure, organogenesis, as well as hormonal and chemical factors.

Ontogeny was considered by Nikolaas Tinbergen, a Dutch scientist who lived in the early 1900s, to be one of the four primary questions which, when answered together, can explain the behaviors, both conscious and instinctive, of a given organism. It was also central to the question of Nature vs. Nurture. Whether an organism's growth and behavior are primarily influenced by its environment or by its genetics.

This science has been practiced in an unofficial or borderline shamanistic manner since ancient times. Historical people were obsessed, as many are today, with making sure that their children grew up strong and free of birth defects. Rudimentary precursors to ontogeny-attempts to direct the physical development of a child-also have historical precedent, such as the Chinese custom of foot binding.

Today, the study of development is carried out via an incredibly wide cadre of tests, observations, and scientific disciplines. Many of these, such as the study of genetics, are staggering, intricate and well-funded fields. These studies are pertinent today from both medical and ethical standpoints. Ontogeny has yielded a lot of information about natural human development, and given doctors the tools and knowledge to spot birth defects and growth anomalies as they happen. Due to modern technology and a long background of ontological studies, scientists can correct more and more genetic faults and lower child mortality.

Ontogeny is also the backdrop of many ethical questions which occupy politicians and laypeople alike. Is an embryo considered a human, and protected by the constitution? What level of development allows for abortion? Soon, as genetic engineering becomes a reality, even more questions will be raised. Should we genetically modify our children? Should we stray from the pattern ontogeny has discovered to make ourselves stronger or smarter?

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