Lecture 9 Topic: Ontogenesis. Biology of development. Types and periods of ontogenesis. Ageing and death.

Plan of lecture:

1. Definition of ontogenesis and phylogenesis.

2. Theory of ontogenesis: Preformation theory and theory of Epigenesis.

3. Types and periods of ontogenesis.

4. Embryonic Period. Embryonic induction.

5. Postembryonic Period.

6. Biological and social aspects of old ageing and death. Gerontology and Geriatry.

1. Ontogenesis (greek: ontos – being, genesis - development) is an individual development of an organism which begins from a fermilized ovum by sperm and zygote is formed by fusion of a sperm and an ovum until death.

Phylogenesis (greek: phyl – breed, generation, genesis - development) is a historical development of an organism or species.

2. Many scientists of the 17^th century thought that either the sperm or the egg cell contained a completely formed, though miniature, human being. In Old Greece Hyppakrat (famous physician) thought that inside of egg formed «a little being». They believed that all the parts were already present, so the embryo had only to grow in size. This concept is known as the Preformation theory.

An opposing view, the theory of epigenesis (greek: epi – after, genesis - development), gained experimental support as better techniques for investigation were developed. Aristotle in Old Medicine, Russian academician K.Wolf are supporters of this theory.

This theory held that the embryo develops from an undifferentiated zygote and that the structure of the body emerge in an orderly sequence, developing their characteristic forms only as they emery.

Today we know that development is largely epigenetic. No invisible «little man» waits preformed in either gamete. Development proceeds from one cell to billions, from a formless mass of cells to a intricate, highly specialized, and organized organism precisely encoded in the form of chemical specifications within the DNA of the genes.

Development is a balanced combination of three processes: growth, morphogenesis, and cellular differentiation.

Growth of the embryo includes both cellular growth and mitosis. The precise and complicated cellular movements that bring about the form of a multicellular organism with its intricate pattern of tissues and organs constitute morphogenesis.

During early development cells begin to differentiate from one another, specializing biochemically and structurally to perform specific tasks. The process by which cells become specialized is known as cellular differentiation.

Types of ontogenesis: direct and indirect.

Indirect type involves development of larva. Same species (namely Insects) characterized by presence in life cycle one or more larval stages. Development of an adult occurs with complete (egg larva pupa imago(adult)) or incomplete metamophosis (egg larva imago(adult)).

Direct type takes place in oviparous and viviparous animals. Oviparous – animals which lay eggs (amphibians, reptiles, birds). Viviparous: such animals give birth to young ones who get nourishment from the mother, e.g. placental mammals.