EGG: STRUCTURE
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 functions: The egg is a biological structure intended by nature for reproduction.
It protects and provides a complete diet for the developing embryo and serves as the principal source of
food for the first few days of the embryo’s life. The egg is the organic vessel containing the zygote in which
an embryo develops until it can survive on its own, at which point the animal hatches. An egg results from
fertilization of an egg cell.
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.
Most arthropods, vertebrates (excluding live-bearing mammals), and mollusks lay eggs, although some,
such as scorpions, do not.
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 larger than 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 granular cytoplasm.
Reptile eggs, bird eggs, and monotreme eggs are laid out of water and are surrounded by a protective shell,
either flexible or inflexible. Eggs laid on land or in nests are usually kept within a warm and favorable
temperature range while the embryo grows. When the embryo is adequately developed it hatches, i.e.,
breaks out of the egg's shell.
An egg basically consists of three parts:
1. a shell
2. an egg white
3. an egg yolk
Ex.: An egg from a hen consists of approximately 2/3 egg white and 1/3 egg yolk.
Structure of an egg
