Structure and Types of Animal egg cell or ovum

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  • Egg cell or ovum, also known as female gamete is formed by the process of oogenesis in female organisms.
  • They are produced by follicle cells present in the ovaries.
  • Egg cells are released in the secondary oocyte stage by the process called ovulation.
  • They are haploid in nature and are obtained by meiosis.
  • Ovum is the largest cell in human beings. They are larger than sperms.


  • Ovum is round (nearly spherical) and is non-motile as it is devoid of a tail.
  • It consists of a central nucleus, cytoplasm and the cell membrane.
  • Nucleus consists of genetic material which once fertilized by sperm, helps to determine the characteristics of the off spring.
  • The cytoplasm surrounding the nucleus is called yolk or ooplasm, which consists of nutrients required for the developing embryo.
  • The peripheral layer of ooplasm is known as cortex which consists of many microvilli and cortical granules.
  • An unfertilized egg in birds and reptiles also consists of outer transparent albumen surrounding the yolk.
  • In telolecithal and mesolecithal eggs, the upper black pigmented half the cell containing a nucleus is called animal pole and the lower whiter half containing yolk is called a vegetal pole.
                 Structure of ovum
  • The egg cell is surrounded by a thin double layered membrane.
  • The inner layer is called vitelline membrane and the outer layer is called zona pellucida.
  • The space between the two layers is called a pre-vitelline membrane.
  • Eggs of birds and reptiles are also covered externally by a hard calcareous shell.
  • On the basis of amount and distribution of yolk, eggs are of various types.

1. On the basis of amount of yolk present, eggs are of following types:

  • Alecithal egg:
    • The ovum consists of almost no yolk in it.
    • Animals like human beings, marsupials and eutherian animals produce such eggs.
  • Microlecithal egg:
    • Such eggs consist of small amount of yolk in them.
    • They are also called oligolecithal eggs.
    • e.g. eggs produced by sea urchin, Branchiostoma
  • Mesolecithal egg:
    • There is moderate amount of yolk present in it.
    • e.g. lungfishes, frogs, toads etc.
  • Megalechital or Macrolecithal or Polylecithal egg:
    • They contain huge amount of yolk in them.
    • e.g. sharks, bony fishes, reptiles, birds, insects and prototherians

2. On the basis of distribution of yolk, eggs are of following types:

  • Homolecithal or Isolecithal egg:
    • Yolk is evenly or uniformly distributed inside an egg.
    • Such distribution is found in alecithal and microlecithal eggs.
  • Telolecithal egg:
    • In such eggs, most of the yolk lies near the vegetal pole.
    • The cytoplasm and nucleus near the animal pole.
    • Such distribution is observed in mesolecithal and macrolecithal eggs.
    • In chicken, the huge amount of yolk occupies the entire ovum but the cytoplasm and nucleus lie in the animal pole. This condition may be referred to as highly telolecithal.
  • Centrolecithal egg:
    • In such eggs, yolk is centrally placed and surrounded by thin cytoplasm.
    • Some insects have centrolecithal eggs.

Structure and Types of Animal egg cell or ovum