![](https://i.imgur.com/dQUR5DO.jpg)
__Photo of a Cell__
[Source](https://pxhere.com/en/photo/770497)
There are many different kinds of cells in the body of an animal or plant. Among these are sex cells. The sex cells of the male animal are different from those of the female. For a new organism to be formed a male sex cell and a female sex cell must join together.
This process is called fertilization. The female sex cells are called eggs or ova and the male sex cells in animals are called sperm.

In plants, they are called pollen cells. Another name for sex cells is gametes.
As a rule in animals, the sex cells of the two different kinds are in different animals-male and female.
Thus, a boy has male sex cells (sperm) and a girl has female sex cells (ova).

There are some animals however in which the two types of cells are contained in one body, and there are no separate male or female animals.
Animals of this type are called hermaphrodites.
The earthworm is a hermaphrodite as it has both sperm and ova in its body. However, the earthworm does not fertilize itself, but sperm from one worm fertilizes the ova of another and vice versa.

In plants, the male and female organs are usually (but not always) in the same plant and sometimes the male cells join with the female cells of the same plant.

As a rule, though, the male cells are carried to the female sex of another by insects, by birds, or by the wind. To sum up, the important thing about sexual reproduction is that there is a union of the male and female gametes. This is called fertilization. From the fertilized egg a new organism results.

## SEX CELLS IN ANIMALS
Animal sex cells are not easy to obtain and look at. The sperm cell is very tiny indeed and can only be seen under a very high powerful microscope. 
In most animals the egg, too, is quite small. Birds are an exception. As you know, a bird's egg is quite large.

The male animal produces an enormous number of sperm which are deposited in the neighborhood of the egg. They gather around the egg, trying to penetrate it. 
![](https://i.imgur.com/5WFNPYc.jpg)
__Process of human fertilization__
source:[Istockphoto](https://www.istockphoto.com/vector/process-of-human-fertilization-gm1151753982-312228527)
The sperm cell has a head and a long tail. With its tail, it can swim about. Of the enormous cluster of sperms, only one manages to penetrate the wall of the egg. The rest die. This one sperm cell unites with the ovum and this is the beginning of a new individual.

*What happens to an egg after fertilization?* Well, the egg then split into two cells. Each of these divides again forming altogether four cells. This process is repeated over and over again until a new organism, made of millions of cells is formed.
![](https://i.imgur.com/FEORnSP.jpg)
__Cell division on a dark background.__
source:[Istockphoto](https://www.istockphoto.com/vector/cell-division-on-a-dark-background-human-anatomy-gm877161410-244793177)
## WATER AS A MEANS OF FERTILIZATION.

If eggs and sperms are stored in different animals, one female, and the other male, how do the sperms get in contact with the eggs to fertilize them? There are several ways in which this is done.

In the case of fish, sperms from the male fish are shed into the water into which the female has already deposited unfertilized eggs. The sperms swing towards the egg and occasionally meet one.

Many of the sperms are swept away by the water and die. To improve the chances of the ova being fertilized the fish sometimes pair off before spawning. The female fish lays her eggs on the bottom of a stream and the male fish comes along and sheds sperms over them.

This gives a greater likelihood of the sperms coming into contact with the eggs and so there is a greater possibility of successful fertilization taking place.

Much the same method is used by amphibians, like the African toad for example. During the dry season, about January, February, and march, you will often hear toads croaking. This is the breeding season, and it is only the male which croaks.

During this time you will sometimes see one toad with another on its back. The female carries the male on her back for 2 or 3 days.
![](https://i.imgur.com/Wq1vHPY.jpg)
__The male is on top of the female. The Fertilization of the eggs of toads are external. The female releases her eggs, which the male frog covers with a sperm solution. The eggs then swell and develop a protective coating.__
Source: [Wikipedia](https://commons.m.wikimedia.org/wiki/File:Male_and_Female_Frog_in_Amplexus.JPG),[CC BY-SA 3.0](https://creativecommons.org/licenses/by-sa/3.0/deed.en).
During this time she lays a change of eggs in water and as she does so the male sheds sperm over them. So there is a very good chance of fertilization taking place.

The eggs are laid in a string of jelly, which is usually fixed to a plant or a rock. 
The function of the jelly is to protect the eggs from mechanical injury when water is disturbed, to separate the eggs, to enable them to receive adequate oxygen, to prevent the bacterial and fungal attack on the eggs, and finally to prevent the eggs from drying up.

In the case of fish and amphibians, they use water to carry the sperms to the eggs. To ensure that some eggs are fertilized, a fish has to produce many eggs.

## FERTILIZATION WITHOUT WATER

*How do animals that live on land bring about fertilization?* They cannot use water to bring the sperms near the eggs. To achieve fertilization the sperms must be deposited inside the egg in the body of the female.
![](https://i.imgur.com/LNoY5fO.jpg)
__Mating of grasshoppers__
[source](https://www.dreamstime.com/mating-grasshoppers-grasshopper-breeds-sexually-namely-distribution-sperm-male-grasshopper-female-body-image198426088)
Look at the picture of locusts. You will see that the male locust is sitting on the back of the female, and if you look closely you will see that the end of the male's abdomen is inside the body of the female. He is injecting sperm into the male's body. There they will meet an egg and one of them will fertilize it.

This is called internal fertilization because the ovum is fertilized inside the female's body. In the case of fish and amphibians fertilization takes place outside the female's body. This is called external fertilization. When the male and the female couple together, as the locusts a coupled in the picture above, we say that they copulate.

Sperms are made by the male reproductive organs, the testes. Below the body of most male land animals through a special part of the abdomen called the penis.

If the sperms are to be deposited inside the female's body there must be some opening through which this happens. This is called the vagina. In copulation, the penis, which becomes stiff, is inserted into the vagina and sperms pass from the penis in a liquid called semen into the female's body. you have probably seen animals such as dogs copulating.
![](https://i.imgur.com/7JxwxFT.png)
__Structure of a sperm__
Source:[Wikipedia](https://en.m.wikipedia.org/wiki/Sperm).
A man produces sperm in the testes. These are two glands that are suspended in a kind of bug called a scrotum between his legs. A boy starts to produce sperm at the age of 12 or 13 and continues to do so for the rest of his life. Unused sperms are destroyed and absorbed by the body.

The eggs are stored in a woman in the ovaries, which is doubled. This is where eggs are stored and it is present in a baby girl when she is born, although, not well developed. At her earlier age possibly when she is about 11 or 12 years old she begins to release these eggs, one about every 28 days. The egg passes down the egg tube or oviduct and enters the womb or uterus.

This process is called ovulation. When an egg is fertilized by sperm introduced by the male it stays in the uterus and develops into a baby. Whenever ovulation takes place, the wall of the uterus becomes spongy and the rich in blood ready to receive the egg if it is fertilized. When the unfertilized eggs break up, the special lining of the uterus wall is no longer needed and it breaks away along with some blood and passes out of the body through the vagina.

This is called menstruation or the 'monthly period', and it starts with girls when their first eggs become mature, which is, as we have said usually about the age of 11 or 12.

When the population takes place the sperm from the male swim up to fertilize the egg in the oviduct. If an ovum is in the oviduct, fertilization will take place, the fertilized egg will pass into the uterus and become embedded in its wall. There the egg cell starts to divide and an embryo is formed.

The embryo depends on its mother for food. It obtains this from a structure in the wall of the uterus called the placenta so which the embryo is joined by a cord called a navel cord (or umbilical cord). Food material passes from the mother's blood flowing to the placenta by a process of diffusion.
![](https://i.imgur.com/1HgJTvp.jpg)
[Image Credit](https://www.freepik.com/premium-vector/fetus-utero-pregnancy-women-diagrams-pregnant-female-anatomy_7679540.htm)
You will see that the growing embryo is well protected inside its mother. Takes 9 months for the embryo to grow big enough and strong enough to face the outside world. This is called a period of gestation. At the end of this time, the child is born. The muscles of the uterus squeeze the child out head first through the vagina. Shortly afterward the placenta is shed. The navel cord is cut, leaving a little lump, which you may perhaps call your 'belly button'.

This is a very wonderful process. like everything else in nature, it is beautifully organized. You will have noticed that a fish produces millions of eggs, but a woman produces comparatively few and sheds them only one at a time.

A woman who is carrying an embryo in a womb is said to be pregnant. She will gradually grow bigger as the embryo develops. You must have noticed this. when the child is born she becomes her former size and shapes again. The same happens, of course, with any other animal which carries its young inside the body for some time before it is born.

## REPRODUCTION IN BIRDS
Birds reproduce sexually, population taking place between the hen bird and the cock. Before this happens there is a period of 'courtship which varies from one kind of birth to another.

Very often the male bird takes on its most brilliant plumage and 'shows it off in front of the hen. With ordinary fowls, the cork moves about with the female usually calling her when he finds food. At this stage the cock is very aggressive and usually drives away any other cork bed that comes near the female. It is very interesting to watch birds of different kinds.
![](https://i.imgur.com/dV81lVd.jpg)
__Mating Birds__
Credit:[pixabay](https://pixabay.com/photos/birds-mating-pigeon-mating-mate-5082940/)
You have probably seen the fowls population in the farmyard or the hen run at your home. The hen bed stops still and lowers her head somewhat, humping her body. The cock hen then mounts on her back.

Before the eggs are fertilized they are already well developed in the ovaries of the hen bird, except that they have no hard shell. Fertilization takes place in the oviduct and the eggs move down. As they do so they are covered with a hard shell. Of course, the eggs passed out of the body of the bed whether or not they are fertilized.

The color of a bird's egg varies. A hen's egg is either white or brown in many cases, the color of the shell is used such as to prevent them from being easily seen by animals that might eat them.

After the hen bird has laid the maximum number of eggs (this varies from one kind of bird to another) she starts sitting on them to keep them warm. During this time the hen often does not eat very much because she does not move about, but sometimes the male bird will bring food to her.

This period is called incubation eggs can be hatched without the help of the mother bird by using an incubator. this is a kind of box that keeps the eggs at the right temperature for them to hatch. In the fowls, the period of incubation is 21 days. After that, the chicks break the shell and are hatched. The mother bird does not feed her young ones once they are hatched; they search for food themselves. With pigeons and swallows, both the male and the female birds look after the young ones.

Many animals that produce young by external fertilization, such as fish, take no care at all of their young ones. They are just left to fend for themselves as soon as they are born. In the process, a large number of them die or are eaten by other animals.

 With the higher animals, and especially with humans, a great deal of care is taken of the young to help them over the difficulties they face when they first come into the world. Young animals are often unable to get food for themselves and have to be fed by their mothers.

In the early days of its life outside the womb, the baby obtains its food by sucking its mother's milk. You will have seen other animals doing this too, such as goats and donkeys. Animals that feed their young in this world are called mammals. The child is kept clean and clothed.

Gradually it becomes able to eat other foods, and it is gradually taking off its mother's milk to more solid foods. Human parents look after their young longer than any other animal does.

## References
•https://courses.lumenlearning.com/boundless-biology/chapter/fertilization/

•https://opentextbc.ca/biology/chapter/24-2-fertilization/

•https://www.britannica.com/science/cross-fertilization
![](https://i.imgur.com/iKFvgQJ.png)
***Thank you for your time spent on reading the article***
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