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Phases of development in humans and some animals which can also be referred to as ontogenesis has several stages and these stages are:

  1.  Gametogenesis: This is the first process leading to the development of a new individual. It starts before fertilization of the egg because the ripening of the egg and the formation of the spermatozoa creates the condition for embryology.
  2. Fertilization: It involves a number of independent biological and physiological processes.
  • The spermatozoa must be brought into proximity with the egg if fertilization is to occur.
  • The spermatozoa must find the egg and penetrate into the egg cytoplasm. This entails a very finely adjusted mechanism of morphological and physio-chemical reactions. The egg is then activated by a spermatozoa and development starts.
  • One of the most important phenomenon after fertilization is the re-arrangement of organ forming substances in the egg.

3. Period of Cleavage : The egg after fertilization by the fusion of the spermatozoa nucleus and that of the egg forms a single cell. If a complex multi-cellular organisms is to develop from a single cell, the later (zygote) must give rise to a large number of cells. This is achieved by a number of mitotic call division following one another in a quick succession. This mitotic division called cleavage cells or blastomeres which becomes smaller and smaller with subsequent division. The result of cleavage is a compact heap of cells. But usually the cells are arranged in a hollo-spherical body called the blastular with a layer of cells the blastoderm surrounding a cavity called the blastocoel (which later forms the stomach or alimentary canal).

4. Gastulation: In this place, the single layer of cells, the blastoderm gives rise to two germinal layer. The germinal layers are complex rudiments from which are derived the various organs of the animal’s body. The animal body consists of 3 layers (Endoderm, Mesoderm, Ectoderm). The Ectoderm give rise to skin  epidemics and the nervous system. The Mesoderm is the source of muscles, the blood, vascular system, the lining of the secondary body cavity and sex organs. The Endoderm forms the alimentary canal and digestive glands.

5. Organogamy: This is also known as organ formation. The continous masses of the cells of the germinal layers becomes split up into smaller groups of cells, each of which is destined to produce a certain organ or part of the animal. Every organ begins its development as a group of cells segregated from the other cells of the embryo. This cells are called the rudiment of the respective organs. This rudiment into which the germinal layer becomes subdivided are called primary organ rudiment.

6. Period of Growth and Histological Development/Differentiation: After the organ rudiment are formed, they begin to grow and greatly increase their volume. In this way, the animal gradually achieve the size of their parents. Sooner or later, the cells in each rudiment becomes histologically differentiated i.e. they acquire the structure, physiological and chemical properties which enable them to perform their physiological functions.

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