Ontogeny

The developmental history includes all the developmental events that occur during the existence of an organism, beginning with the changes in the egg at the time of fertilization and events from the time of birth or hatching and afterward (i.e., growth, remolding of body shape, development of secondary sexual characteristics, etc.).

Aspects of ontogeny are morphogenesis, the development of form and shape of an organism; tissue growth; and cellular differentiation.

Haeckel suggested that ontogeny briefly and sometimes incompletely recapitulated or repeated phylogeny in his 1866 book, Generelle Morphologie der Organismen ("General Morphology of Organisms").

Even though his book was widely read, the scientific community was not very convinced or interested in his ideas, so he turned to producing more publications to get more attention.

[10] A seminal 1963 paper by Nikolaas Tinbergen named ontogeny as one of the four primary questions of biology, along with Julian Huxley's three others: causation, survival value and evolution.

Each species of animal has a slightly different journey through these stages, since some stages might be shorter or longer when compared to other species, and where the offspring develops is different for each animal type (e.g., in a hard egg shell, uterus, soft egg shell, on a plant leaf, etc.).

These outer cells form a single epithelial layer, the blastoderm, that essentially encases the fluid-filled inside that is the blastocoel.

[20] The endoderm cells become the internal linings of organisms, such as the stomach, colon, small intestine, liver, and pancreas of the digestive system and the lungs.

This figure shows how the germ layers can become different organs and tissues in evolutionarily higher life-forms and how these species essentially develop very similarly.

Next, the notochord degenerates to become only the nucleus pulposus of the intervertebral discs and the mesoderm cells differentiate to become the somites and skeletal muscle later on.

Also during this stage, the neural crest cells become the spinal ganglions, which function as the brain in organisms like earthworms and arthropods.

This process initiates once primary neurulation is finished and the posterior neuropore closes, so the tail bud can proliferate and condense, then create a cavity and fuse with the central canal of the neural tube.

[23] Secondary neurulation is seen in the neural tube of the lumbar and tail vertebrae of frogs and chicks and in both instances, this process is like a continuation of gastrulation.

Caterpillars keep growing and feeding in order for enough energy during the pupal stage, when necessary body parts for metamorphosis are grown.

[29] In animals, the juvenile stage is most commonly found in social mammals, such as wild dogs, monkeys, apes, lions, wolves, and more.

The process of an organism's body undergoing structural and physical changes after birth or hatching to become suitable for its adult environment is metamorphosis.

[31] For example, amphibian tadpoles have a maturation of liver enzymes, hemoglobin, and eye pigments, in addition to their nervous, digestive, and reproductive systems being remodeled.

physically mature faster than large breeds (e.g., Saint Bernard, Great Dane, Golden Retriever, etc.

Even reptiles (non-avian sauropsids, e.g., crocodilians, turtles, snakes,[39] and lizards[40]), in which the offspring are often viewed as miniature adults, show a variety of ontogenetic changes in morphology and physiology.