Contents
Molecular biology, one of the Biology Topics, focuses on the study of biological molecules.
What is Fertilisation and Types of Fertilization in Sexual Reproduction ?
All the living organisms grow old with time and ultimately die. In fact, every living organism remains alive on this earth for a limited period of time and then dies. So, new organisms have to be produced in place of those who die. The production of new organisms from the existing organisms of the same species is known as reproduction.
In most simple words we can say that reproduction is the creation of new living things (from the existing living things). Actually, one of the most important characteristics of living organisms is their ability to reproduce more members of their species. Reproduction is essential for the survival of a species on this earth. So, living organisms produce more organisms of their kind to maintain the life of their species on this earth.
The process of reproduction ensures continuity of life on earth. For example1 human beings reproduce by giving birth to babies (sons and daughters). These babies grow and ultimately become adults. So, when the old parents die, their sons and daughters keep living on this earth.
These sons and daughters also reproduce by giving birth to more babies, and this process goes on and on. Thus, reproduction by human beings ensures that the human species will continue to exist on this earth for all the time to come. Similarly, cats reproduce by giving birth to kittens so that their species may live forever.
And dogs reproduce by giving birth to puppies so that their species may continue to live on this earth.
It is clear from the above discussion that for a species of an animal to continue living on this earth, it must reproduce itself. Reproduction gives rise to more organisms with the same basic characteristics as their parents. For example, human beings always produce human babies; cats always produce kittens; and hens always produce chicks. If, however, some species of the living organisms cannot reproduce due to certain reasons, then the organisms of this species will gradually die out and disappear from this earth one day. In this chapter we will discuss the various methods of reproduction in animals.
Before we go further, we should know the meaning of the term ‘young one’ of an animal. The newly bom animal (or newly hatched animal) is called young one. The young ones of different animals have different names. For example, the young one of humans is called ‘baby’ the young one of cat is kitten, the young one of dog is puppy, the young one of cow is calf, the young one of horse is colt, the young one of lion is cub, the young one of hen is chick, the young one of frog is tadpole, and the young one of a butterfly is caterpillar. The young one of an animal is also called its offspring.
There are many different ways in which new organisms are produced from their parents. Some organisms like Amoeba just split into two parts to produce new Amoebae; some organisms like Hydra grow out of the parent’s body in the form of a bud; some organisms like birds and snakes hatch out of the eggs laid by their parents; whereas some organisms like human babies, kittens and puppies are born from their mother.
This means that each species of organisms reproduces in a different way. All the different ways of reproduction can be divided into two main groups: asexual reproduction and sexual reproduction. Thus, there are two main methods of reproduction in living organisms:
- asexual reproduction, and
- sexual reproduction.
This means that new living organisms (or animals) can be made either by the method of ‘asexual reproduction or by the method of‘sexual reproduction’. We will now discuss the meaning of asexual reproduction and sexual reproduction. In order to understand this please keep in mind that certain organisms contain ‘reproductive cells’ (called ‘sex cells’ or ‘gametes’) in their bodies whereas some other organisms do not contain ‘reproductive cells’ (‘sex cells’ or gametes’) in their bodies.
1. Asexual Reproduction
The production of a new organism from a single parent without the involvement of sex cells (or gametes) is called asexual reproduction. It is called asexual reproduction because it does not use special cells called ‘sex cells’ (or gametes) for producing a new organism. In asexual reproduction, a part of the parent organism separates off and grows into a new organism. Thus, in asexual reproduction, only one parent is needed to produce a new organism. But no sex cells are involved in asexual reproduction. Some of the examples of asexual reproduction are: binary fission in Amoeba; and budding in Hydra. Please note that asexual reproduction is the simplest method of reproduction. It takes place mainly in those animals whose bodies have a simple structure. In asexual reproduction, the young one (or offspring) produced is an exact copy of the parent.
2. Sexual Reproduction
The production of a new organism from two parents by making use of their sex cells (or gametes) is called sexual reproduction. In sexual reproduction, the sex cell of one parent fuses with the sex cell of the other parent to form a new cell called ‘zygote’. This zygote then grows and develops to form a new organism. Thus, in sexual reproduction, two parents are needed to produce a new organism. The two parents which are involved in sexual reproduction are called male and female. The male and female parents have special organs in them which produce male sex cells and female sex cells respectively (which are required in sexual reproduction).
The humans, fish, frogs, cats and dogs, all reproduce by the method of sexual reproduction. In sexual reproduction, the young one (or offspring) produced is not an exact copy of the parents.
The basic difference between asexual and sexual reproduction is that only one parent is needed in asexual reproduction whereas two parents are needed in sexual reproduction. Another difference is that no sex cells (or gametes) are involved in asexual reproduction but sex cells (or gametes) take part in sexual reproduction. In order to understand the sexual reproduction in animals, we should know the meanings of some important terms like male sex, female sex, gametes, sperms, eggs (or ova), fertilisation, and zygote which are involved in sexual reproduction. These are discussed below.
Male and Female
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. Just like us human beings, other animals also 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 ‘eggs’ (or ‘ova’) in its body is called female. We will now discuss gametes.
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 gametes’. We can now say that: The cells involved in sexual reproduction are called gametes. 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 ‘egg’ (or ‘ovum’). We will now describe sperms and eggs (or ova) in somewhat detail.
(i) SPERMS. The sperms are extremely small cells and we need a microscope to see them. A sperm is about 0.05 mm long. A highly enlarged sketch of sperm is shown in Figure. A sperm has a head, a middle piece and a tail. A sperm is a single cell with all the usual cell components like nucleus, cytoplasm and cell membrane. The nucleus of sperm cell is tightly packed in its head. The sperm cell has very little cytoplasm. The purpose of tail of sperm is to make it move. The sperm moves by waving its tail from side to side. The term ‘sperm’ is the short form of the word ‘spermatozoon’ (which means male sex cell or male gamete).
(ii) EGGS (OR OVA). The eggs (or ova) are also very small and we need a microscope to see them. Eggs (or ova) are, however, much larger than the sperms. The human egg (or ovum) is round and about 0.15 mm in diameter. A highly enlarged sketch of an egg (or ovum) is shown in Figure. The egg (or ovum) is also a single cell having a nucleus, cytoplasm and a cell membrane. Outside the cell membrane, an egg (or ovum) has a thin layer of jelly called ‘jelly coat’ which allows only one sperm to enter into it during fertilisation. Please note that a female gamete (or female sex cell) is known by two names: ‘egg’ and ‘ovum So, whether we use the name ‘egg’ or ‘ovum it will mean the same thing. Another point to be noted is that the plural of ovum is ova. Some of the animals like birds (hen, ostrich, etc.) and reptiles (snakes, crocodiles, etc.) have large eggs.
The nuclei of sperm and egg contain chromosomes which carry genes and transmit the characteristics of parents to the offsprings. The fusion of a male gamete (sperm) with a female gamete (egg) gives rise to a new cell called zygote. Thus, the new cell which is formed by the fusion of a male gamete and a female gamete is called zygote. Zygote is a single cell which contains one nucleus. The nucleus of zygote is formed by the combination of nuclei of ‘sperm’ and egg’. The process of fusion of gametes is called fertilisation. This is discussed below.
Fertilisation
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 sexual reproduction, is called fertilisation. Since the male gamete of an animal is called sperm and the female gamete of an animal is called egg (or ovum), therefore, we can also say that: The fusion of a sperm with an egg (or ovum) to form a zygote during sexual reproduction, is called fertilisation. Thus, the process of fertilisation produces a new cell called zygote. The zygote is actually a fertilised egg (or fertilised ovum).
In human beings, the process of fertilisation is the meeting of a sperm cell from the father with an egg cell from the mother to form a fertilised egg cell called zygote. The process of fertilisation in humans is shown in Figure. Figure (a) shows a sperm and an egg. Figure 3(b) shows the head of a sperm entering the egg.
Please note that though many sperms try to enter the egg, only one sperm is successful in entering the egg (We have shown only one sperm in Figure to keep the diagram simple). When the head of sperm enters the egg, then the nucleus of sperm cell fuses (or joins) with the nucleus of egg cell to form a new nucleus. Figure (c) shows the fusion of nucleus of sperm with the nucleus of egg to form a new cell called ‘fertilised egg’ or ‘zygote’ (The tail of successful sperm entering the egg remains outside). All the multicellular animals (including humans) start their life from a single cell called zygote (through sexual reproduction).
Internal Fertilisation and External Fertilisation
We have just studied that the fusion of a sperm with an egg is called fertilisation. Now, the egg (or ovum) is made in the body of a female animal. So, the fertilisation of an egg by a sperm can take place either inside the body of the female animal or outside the body of female animal. This leads to two modes of fertilisation in animals : internal fertilisation, and external fertilisation.
The fertilisation which takes place inside the female body is called internal fertilisation. In internal fertilisation, the female animal’s eggs are fertilised by sperms inside her body. In internal fertilisation, the male animal puts his sperms into the female animal’s body. And these sperms then fertilise the eggs inside her body. For example, a man puts his sperms inside a woman’s body. These sperms then fertilise the egg inside the woman’s body. This type of fertilisation in which the fusion of a male gamete (sperm) and a female gamete (egg) occurs inside the body of the female animal, is called internal fertilisation.
Internal fertilisation takes place in a very large number of animals such as humans, cows, dogs, cats, tigers, lions, rabbits, deer, horse, birds (such as hen, sparrow, crow, pigeon, etc.), reptiles (such as lizard, snake and crocodile, etc.), and insects (such as silk moth, housefly and butterfly, etc.). In all these animals, fertilisation occurs inside the body of the female animal. The fertilisation which takes place outside the female body is called external fertilisation. In external fertilisation, the female animal’s eggs are fertilised by sperms outside her body. In frogs and most fishes, the fertilisation of eggs occurs outside the female animal’s body.
In external fertilisation, the male and female animals release their sperms and eggs in water where fertilisation takes place by collisions between sperms and eggs. For example, the males and females of frogs and fishes release their sperms and eggs in water in which they live. The sperms then collide with the eggs and fertilise them outside the body of female frog or fish. External fertilisation is very common in aquatic animals such as frog, fish and star fish, etc. In these animals, fertilisation takes place in water. We will now describe the external fertilisation in frogs in somewhat detail.
During spring season (or rainy season), frogs move to the water in ponds and slow-moving streams. When the male frog and female frog come together, the female frog lays hundreds of eggs in water (see Figure).
Unlike the hen’s eggs, frog’s eggs are not covered by hard shells. The frog’s eggs are very delicate. A layer of jelly holds the frog’s eggs together and provides protection to the eggs. When the female frog lays hundreds of eggs, at the same time the male frog releases millions of sperms from its body (see Figure). The sperms swim randomly in water with the help of their long tails. When the sperms of frog come in contact with the eggs floating in water, then fertilisation takes place. This type of fertilisation in which the fusion of a male gamete (sperm) and a female gamete (egg) takes place outside the body of the female animal, is called external fertilisation.
Just like frogs, external fertilisation also takes place in fish. When the male fish and female fish come together in water, the female fish lays hundreds of eggs in water and at the same time, the male fish releases millions of sperms in water. When the sperms come in contact with eggs, the eggs get fertilised. In this way, a female fish’s eggs are fertilised by sperms outside its body. This is also an example of external fertilisation.
We will now explain why the animals like frog and fish lay hundreds of eggs at a time (whereas a hen lays only one egg at a time). Though the animals like frog (and fish) which undergo external fertilisation, lay hundreds of eggs, all the eggs do not get fertilised and hence do not develop into new frogs (or fishes). This is because of the following reasons :
- Frog (and fish) lay eggs and release sperms in water. Many of these eggs and sperms are carried away by the movement of water, wind or rainfall, and hence all the eggs do not get fertilised.
- Many of the eggs of frogs (and fish) are eaten up by other animals which live in water.
- Many of the larvae (young ones) of frog and fish which are hatched from the fertilised eggs are eaten up by other animals in water and fail to develop into adult frogs (or fish).
So, out of hundreds of eggs laid by the female frog (or female fish), only a few will survive to become adult frogs (or adult fish). Since many of the eggs of frogs and fish are carried away by moving water, wind or rain, and many are eaten up by other animals, therefore, the production of a large number of eggs is necessary to ensure the fertilisation of at least a few of them. Out of hundreds of eggs laid by female frog (or female fish) only a few get fertilised and their larvae survive to become adult frogs (or adult fish).
We know that the new cell which is formed by fertilisation is called ‘zygote’. And this zygote then grows and develops into a full organism (or baby animal). The method in which a zygote grows and develops into a full organism also varies in different animals. For example, in human beings the zygote grows and develops into a baby inside the female body (mother’s body). And then the mother gives birth to the baby. Just like humans, the animals like cats and dogs also give birth to their young ones.
But the process is entirely different in those animals (or birds) which lay eggs. For example, a hen sits on its fertilised eggs for a considerable time to give them warmth. During this period, the zygote grows and develops to form a complete chick. This chick then comes out of the egg by breaking its shell. It is clear from this discussion that all the organisms do not give birth to individuals like humans do.