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the formation and development of organs in animals. Ontogenetic organogenesis is studied through embryology and biology of development, while phylogenetic organogenesis is studied through comparative anatomy. These disciplines describe and analyze the processes of organogenesis and explain the phylogenetic and ontogenetic origins of the processes. Comparative anatomy examines the rise of new organs and the transformation, division, progressive development and reduction, and rudimentation of organs. Study of the development of the forms of organs in connection with organ function led to the discovery of the basic principles of phylogenetic organogenesis—differentiation (seeDIFFERENTIATION), integration (seeINTEGRATION), and change of function.
To a large extent, ontogenetic organogenesis repeats phylogenetic organogenesis (seeBIOGENETIC LAW). Successive differentiation and integration of organs and uneven growth and active migration of cellular material occur in the course of ontogenetic organogenesis. The forces that underlie ontogenetic organogenesis can be precisely studied, especially by experiment. (For initial and subsequent stages of organogenesis seeDETERMINATION, CLEAVAGE, EMBRYONIC DEVELOPMENT, GERM LAYERS, INDUCTORS, INDUCTION, ORGANIZER.)
With respect to plants, “organogenesis” usually refers to the ontogenetic formation and development of basic organs—roots, stem, leaves, and flowers—from undifferentiated tissue, or meristem (seeMERISTEM).