The Structure of a typical Ovum

Ovum is the female gamete. lt stores food required for the entire process of development in the form of yolk. lt has three important f unctions:

1. lt supplies a haploid set of chromosomes to the future embryo.

2. lt contributes almost all cytoplasm to the zygote.

3. lt supplies food to the developing embryo.

Shape and Size

Typically, the eggs are spherical or ovoid in shape. But in a few animals like insects, the eggs are elongated and cylindrical in nature. Eggs are generally large rthan the sperms and average somatic cells. The size of a mature egg depends on the amount of yolk present in it. The smallest known egg is that of mouse (0.07mm); the birds possess larger eggs. Ostrich lays the largest egg having a diameter of about 85 mm.

The egg is covered externally by a plasma membrane or plasmalemma. Within the plasma membrane is the granularcytoplasm

Organisation of Egg Cytoplasm

The cytoplasm of egg cell is known as ooplasm. lt is granular and contains in addition to the usual cellular organelles certain other inclusions Iike yolk, pigments and cortical granules. The peripheral layer of ooplasm is more viscous and gelatinous. lt is known as the egg cortex which is provided with many microvilli and cortical granules. The microvilli are formed by the outpushings of the plasmalemma and they help in transportation of substances from the outside into the ooplasm during the development of egg. The cortical granules are very small spherical bodies varying in diameter from 0.8 µm to 2µm . They are

rnembrane bound and are formed from golgi complex. They contain homogeneous and granular mucopolysaccharides. Cortical granules are present in the eggs of sea urchins, frogs, fishes, bivalve molluscs, several annelids and certain mammals.

Yolk:Nutritive substances are stored in the cytoplasm of egg in the form of yolk or deutoplasm. This stored food is utilized by the embryo for its early development. The process of formation of yolk is known as vitellogenesis. The yolk is a complex material consisting of proteins, fats, carbohydrates, inorganic salts, vitamins, enzymes, pigments and water. The yolk may be called "protein yolk" when it has more proteins than lipids, or " fatty yolk" when it has more fat contents than the proteins. Most animal eggs contain both kinds of yolk. Since the yolk is heavier ,large quantities of yolk, such as those of the frog and chick, the accumulation of yolk in one region is so marked that they are known as telolecithal eggs. In eggs containing lesser amount of yolk, like those of Amphioxus and man, the york is distributed more uniformry, hence they are known as isolecithatal or  homolecithal.

Pigment granules are present in the cytoplasm of eggs of many species. The granules may be brown, black, red, yellow, green or- grey in colour. As the pigment granules are not common to all eggs, they do not play any significant role in development.

Polarity

The constituents of egg are not uniformly distributed throughout the cytoplasm. These are distributed in such a way that two poles distinct can be identified in the egg. These pores are known as animal pole and vegetal pore. The cytoplasm is concentrated in the upper portion or animal hemisphere and the yolk material is concentrated in the lower portion or vegetal hemisphere. A plane passing through these two poles constitute the polar axis. The nucleus is always located in the polar axis, more or less towards the animal pole. The yolk shows a gradation from the animal pole towards the vegetal pole. There is also a metabolic gradation along the polar axis. Metabolic processes are highest at the animal pole and progressively diminish towards the vegetal pole