In order to understand sexual reproduction, we should know the meanings of some important terms like male sex, female sex, gametes, sperms, ova (or eggs), fertilisation, zygote and embryo, which are involved in sexual reproduction. These are discussed below:
Our father is a male and our mother is a female. We can also say that our father has male sex and our mother has female sex. Now, our father is a man and our mother is a woman. This means that a man is male whereas a woman is female.
Thus, a man is said to have male sex and a woman is said to have female sex. Similarly, a boy has a male sex and a girl has a female sex. Just like us human beings, other animals also have male and female sexes.
Even the plants have male and female sexes. As we will learn after a while, being male or female depends on the type of sex cells present in one’s body. An animal having male sex cells called ‘sperms’ in its body is called male. On the other hand, an animal having female sex cells called ‘ova’ (or ‘eggs’) in its body is called female. We will now discuss gametes.
Sexual reproduction takes place by the combination of special reproductive cells called ‘sex cells’. These sex cells are also known by another name which is ‘gamete’. We can now say that: The cells involved in sexual reproduction are called gametes.
In other words, gametes are the sexual reproductive cells (or sex cells). Gametes are of two types: male gametes, and female gametes. The male gamete in animals is called ‘sperm’ and the female gamete in animals is called ‘ovum’ or ‘egg’. Sperms and ova (or eggs) are extremely small cells which can be seen only with the help of a high power microscope. Please note that a female gamete (or female sex cell) is usually known by two names: ovum and egg.
So, whether we use the term ‘ovum’ or ‘egg’, it will mean the same thing. Another point to be noted is that the plural of ovum is ova. The ovum or egg contains water and stored food. The important part of ovum or egg is its nucleus. The sperm cell is hundreds or even thousands of times smaller than the ovum or egg and it has a long tail. The sperms are motile which can move independently with the help of their tails. The nuclei of the sperm and ovum (or egg) contain chromosomes which carry the genes.
It is clear from the above discussion that sperms are the male gametes of animals. And ova (or eggs) are the female gametes of animals. In other words, sperms are the male sex cells of animals and ova (or eggs) are the female sex cells of the animals.
As we will study after a short while, fusion of gametes gives rise to a single cell called zygote. We can also say that the cell which is formed by the fusion of a male gamete and a female gamete is called zygote. In most simple words, zygote is a ‘fertilised ovum’ or ‘fertilised egg’. All the multicellular animals start their life from a single cell called zygote through sexual reproduction. The process of fusion of gametes is called fertilisation. This is discussed below.
For sexual reproduction to occur, a male gamete must combine (or fuse) with a female gamete. The fusion of a male gamete with a female gamete to form a zygote during the sexual reproduction is called fertilisation.
Since the male gamete of an animal is called sperm and the female gamete of an animal is called ovum (or egg), therefore, we can also say that: The fusion of a sperm with an ovum (or egg) to form a zygote during sexual reproduction, is called fertilisation. It is clear that the process of fertilisation produces a new cell called zygote.
The zygote is actually ‘fertilised ovum’ or ‘fertilised egg’. The zygote (or fertilised egg) grows and develops to form a new baby. The unborn baby in the uterus in the early stages of development (when its body parts are not much developed) is called an embryo.
On the other hand, the unborn baby in the uterus in the later stages of development (when all its body parts are well developed and can be identified) is called a foetus (The word ‘foetus’ is pronounced as ‘fetus’).