Ontogenesis – individual development

Ontogenesis is the individual development of an organism: the sum of morphological, physiological and biochemical changes from the formation of the zygote (in asexual reproduction, from separation from the parent) to the end of life. Three types are distinguished: larval (insects, fish, amphibians – metamorphosis in the larva), development in an egg (reptiles, birds – much yolk) and development in the mother’s body (higher mammals, through the placenta). In the embryonic period the zygote cleaves by mitosis (blastomeres → blastula, a hollow ball of one layer); in gastrulation a two-layered gastrula forms (ectoderm, endoderm) and then a third layer, the mesoderm; in organogenesis the neural tube, notochord and gut tube appear: the nervous system and epidermis develop from ectoderm, muscle, bone and the circulatory system from mesoderm, and the lining of the digestive and respiratory tracts from endoderm. Cells specialise (differentiation): some genes are active and others switched off. The postembryonic period includes juvenile, mature and ageing stages; development is direct or with metamorphosis (complete: egg–larva–pupa–adult; incomplete: egg–larva–adult). Homeostasis, regeneration and immunity keep the organism stable; biorhythms (daily, seasonal – photoperiodism) arose through natural selection; in anabiosis metabolism slows sharply (seeds, spores, cysts).