Reproduction in Animals
1.0Introduction
Reproduction means 'to reproduce', to produce more of the same kind, to perpetuate the species. Without reproduction a species would ultimately die out. The phenomenon of producing new individual from existing ones to increase the community is called reproduction.
- In previous classes, we have learnt that reproduction is very important as it ensures the continuation of similar kinds of individuals, generation after generation. Every existing plant, animal and other living organisms are the result of reproduction.
Modes of Reproduction: The ability to produce more members of the same species is considered as the most important characteristic of living organisms. It is important for the survival of an individual organism on the earth. For example, human beings give birth to their young ones. In the same manner, other organisms give birth to their young ones.
There are two modes by which animals reproduce. These are - (i) Sexual reproduction (ii) Asexual reproduction
2.0Sexual Reproduction
It is a mode of reproduction that involves fusion of female gamete and male gamete. Sexual reproduction in flowering plants is accomplished through the agency of flower because both male and female reproductive parts are located in it. In animals, males and females have different reproductive parts or organs. These reproductive parts of two parental organisms produce special types of cells called as gametes that fuse to form zygote, which develop into a new individual (offspring). The mode of reproduction in humans is sexual. In humans, there are two types of reproductive system.
- Male reproductive system
- Female reproductive system
Male Reproductive System
Male reproductive system includes - (i) A pair of testes (ii) A pair of sperm ducts (iii) A penis
Male Reproductive Organs in Humans
(i) Testes: The testes produce the male gamete called sperm. Testes lie in the scrotal sac. Millions of sperms (male gametes) are produced within the testes every day. Scrotal sacs keep the testes temperature at lower than the body temperature. Sperm: A sperm is a single celled male gamete with three parts: (a) Head: contains nucleus. (b) Middle piece: contains numerous mitochondria. (c) Long tail: helps in movement. All the usual cell components are present in the sperm. The sperms are motile because of long tail.
Human sperm
(ii) Sperm duct: They are also known as vas deferens. Two sperm ducts through which sperms leave the testes and reach to penis.
(iii) Penis: The muscular organ having a drainage duct through which urine and sperm are pushed out.
Female Reproductive System
The female reproductive system includes - (i) A pair of ovaries (ii) A pair of oviducts (fallopian tube) (iii) Uterus
Female Reproductive Organs in Humans
(i) Ovaries: The ovaries lie in the lower part of the abdominal cavity. In human beings, a single matured egg is released into oviduct by one of the ovaries every month. Egg is also known as ova (singular ovum) or female gamete. This egg passes through the fallopian tube to reach the uterus. An egg is a single cell.
(ii) Oviduct: Oviduct is also called fallopian tube. Each oviduct has funnel shaped opening which helps to pick up and push the released ovum into the oviduct. The process of releasing the ovum by female is called ovulation.
- Oviducts/fallopian tubes are the site of fertilization.
(iii) Uterus: It is muscular structure commonly known as womb. The uterus acts as a life support system. The place where fertilized ovum (zygote) gets implanted (fixed), receives nourishment and develops into an infant.
Q. Both sperm and ovum are extremely small cells but why ovum is larger than sperm?
- Explanation Ovum is larger in size than sperm because of the presence of water and stored food.
Fertilization
(i) For sexual reproduction to occur, a male gamete should fuse with a female gamete. (ii) The fusion of male gamete and female gamete to form a zygote during sexual reproduction is called fertilization. (iii) When sperms come in contact with egg, one of the sperm may fuse with the egg. (iv) During fertilization the nuclei of the sperm and the egg fuse to form single nucleus. (v) This results in the formation of zygote or fertilized ovum. (vi) So new individual inherits some characters from father and some from mother.
Fertilization
3.0Types of Fertilization
(i) Internal fertilization: The type of fertilization which occurs inside the body of female is called internal fertilization. It occurs in many animals including human, cow, dogs and hens. Internal fertilization in Hens: Both in humans and hens, internal fertilization takes place, but humans give birth to babies whereas hens do not. Hens lay eggs which give rise to chickens as their young ones.
After fertilization zygote divides repeatedly. At the same time a protective layer is formed around the developing embryo. The laying of the eggs by the hen takes place after the hard shell is formed around the developing embryo. The development of the chick takes place inside the eggshell and it takes about three weeks. The sitting of a hen on the eggs is to provide warmth to the eggs. As soon as the chick is developed, it comes out by bursting and open the eggshell.
Hen with eggs
(ii) External fertilization: Fertilization which occurs outside the body of female is called external fertilization. In frogs and toads, reproduction activity starts during spring or rainy season. These animals move to ponds and slow-moving streams. The male and female come together in water. The female lays hundreds of eggs which are held together by a layer of jelly. The jelly provides protection to the eggs beside holding them together. The male deposits sperms over the eggs. The sperms after coming in contact with the eggs bring about fertilization. The type of fertilization in which fusion of male and female gametes takes place outside the body of female is called external fertilization. It is very common in aquatic animals such as fish, starfish etc. Animals which undergo external fertilization (e.g. frog), embryos continue to grow within their coverings. After the development, the eggs hatch and numerous tadpoles can be seen swimming in ponds and streams.
Q. Why do fishes and frogs lay eggs in hundreds whereas a hen lays only one egg at a time?
- Explanation External fertilization is found in these animals. Though these animals lay hundreds of eggs and release millions of sperms, all the eggs do not get fertilized and develop into new individuals. This is because the eggs and sperms get exposed to water movement, wind and rainfall. Also, there are other animals in the pond which may feed on eggs. Thus, production of large number of eggs and sperms is necessary to ensure fertilization of at least a few of them. Hen does not need to produce large number of eggs because of internal fertilization. They are not exposed to the environment to get destroyed.
Development of embryo:
Zygote formed after fertilization begins to develop into an embryo. It starts dividing repeatedly to produce a hollow ball of cell. The cell begins to form a group that develops into different tissues and organs of the body. This developing structure is termed as embryo. The embryo attaches to the wall of uterus or gets embedded in the wall of uterus for further development. Inside the womb the umbilical cord connects the baby with mother's placenta which provides nutrition to the foetus. Here it develops over the nine months. The developing embryo gradually differentiates into different body parts such as hands, legs, head, eyes, ears etc. The stage of the embryo in which all the body parts can be identified is called as foetus.
(a) zygote formation and development of an embryo from the zygote; (b) Ball of cells (enlarged); (c) Embedding of the embryo in the uterus (enlarged).
Gestation period: It is the period from fertilization to birth. In human beings, it is of about nine months (270 days). In this period growth and development of embryo inside the womb of the mother take place.
Foetus inside mother's
Implantation: Attachment of fertilized egg to the lining of uterus.
Placenta: An organ that connects the developing foetus to uterine wall to allow nutrient uptake, waste elimination & gas exchange through mother's blood supply.
Q. What do you mean by IVF? Why the baby born by this process is called test tube baby?
- Explanation In some women oviducts are blocked. These women are unable to bear babies because sperms cannot reach the egg for fertilization. In such cases, doctors collect freshly released egg and sperms and keep them together for a few hours for IVF or In vitro fertilization (fertilization outside the body). In case fertilization occurs, the zygote is allowed to develop for about a week and then it is placed in the mother's uterus. Complete development takes place in the uterus and the baby is born like any other baby. So babies born through this technique are called test-tube babies.
In vitro fertilization
Viviparous and Oviparous Animals
Most animals lay eggs while some animals give birth to young ones. The animals which give birth to young ones are called viviparous animals. Those animals which lay eggs are called oviparous. The eggs of the few animals are easy to collect because their mothers lay them outside their body. These are called oviparous animals, for e.g. insects, frogs, snakes, turtles, butterfly and birds (hen, crow etc.).
But the egg collection of some animals, is not possible because in these animals, mothers give birth to young ones. These are viviparous animals, for e.g. dog, cat, cow, elephant and human beings. The process of laying egg in large number is called spawning.
Young Ones to Adult
All egg laying animals are called oviparous animals. The new individual which are born or hatched from the egg, continue to grow till they become adult. The transformation of the larva into adult through drastic changes is called metamorphosis. We can recapitulate the life cycle of the silkworm.
For example, there are different stages in the life cycle of frog starting from the egg to adult Egg tadpole larva adult. A frog lays eggs in the large number. The baby that hatches out of a frog's egg is called tadpole. The tadpole undergoes several changes before it becomes an adult frog. The features that are present in adult are not found in young ones. The tadpoles transform into adult capable of jumping and swimming.
4.0Asexual Reproduction
There are many ways in which new organisms are produced from their parents. Some microscopic organisms like Amoeba and Hydra reproduce asexually. Yeast (unicellular fungi) reproduces asexually by budding. In asexual reproduction, the offspring arises from single parent. The production of a new organism from a single parent without the involvement of gametes is called asexual reproduction. Types of asexual reproduction: There are many types of asexual reproduction. Some of them are as following -
Budding
Budding is commonly seen in Hydra. (i) In this type of reproduction, one or more small bulb like outgrowth or bulges are formed on the body. (ii) These are called buds and are responsible for giving rise to new individual. (iii) The bud detaches itself from the parent body. It grows to full size and becomes a new individual.
This method of asexual reproduction in which individual develops from the bud is called budding.
Budding in Hydra
Binary Fission
The type of asexual reproduction in which an animal reproduces by dividing into two individuals is called binary fission. It begins by division of its nucleus into two nucleus, followed by division of its body. E.g.: Amoeba, Paramecium, Bacteria etc.
Cloning
Cloning is the production of an exact copy of a cell, any other living part, or a complete organism. Cloning of an animal was successfully performed for the first time by Ian Wilmut and his colleagues at the Roslin Institute in Edinburgh, Scotland. They successfully cloned a sheep named Dolly. Dolly was born on July 1996 and was the first mammal to be cloned.
During the process of cloning Dolly, a cell was collected from the mammary gland of a female Finn Dorsett sheep. Simultaneously, an egg was obtained from a Scottish blackface ewe. The nucleus was removed from the egg. Then, the nucleus of the mammary gland cell from the Finn Dorsett sheep was inserted into the egg of the Scottish blackface ewe whose nucleus had been removed. The egg thus produced was implanted into the Scottish blackface ewe. Development of this egg followed normally and finally Dolly was born. Though Dolly was given birth by the Scottish blackface ewe, it was found to be absolutely identical to the Finn Dorsett sheep from which the nucleus was taken. Since the nucleus from the egg of the Scottish blackface ewe was removed, Dolly did not show any character of the Scottish blackface ewe. Dolly was a healthy clone of the Finn Dorsett sheep and produced several offsprings of her own through normal sexual means. Unfortunately, Dolly died on February 2003 due to a certain lung disease. Since Dolly, several attempts have been made to produce cloned mammals. However, many die before birth or die soon after birth. The cloned animals are many-a-times found to be born with severe abnormalities.
5.0Some Important Points
- The eggs we get from the market are generally the unfertilised ones. To observe a developing chick embryo, get a fertilised egg from the poultry or hatchery which has been incubated for 36 hours or more. A white disc-like structure on the yolk. This is the developing embryo. Sometimes if the heart and blood vessels have developed you may even see a red spot.
- Hydra is transparent, jelly-like organism with tentacles.
- An interesting organisation is observed in a honey bee hive, a colony of several thousand bees. Only one bee in the colony lays eggs. This bee is called the queen bee. All other female bees are worker bees. Their main job is to build the hive, look after the young and feed the queen bee adequately to keep her healthy so that she can lay eggs. A queen bee lays thousands of eggs. The fertilised eggs hatch into females, while the unfertilised eggs give rise to males, called drones. It is the job of the worker bees to maintain the temperature of the hive at around to incubate the eggs.
- Identical twins: Twins those are develop from one zygote after splitting of that one zygote. So identical twins are always of the same sex.
- Non identical twins: Twins those are develop from two different zygote at the same times.
6.0Chapter At A Glance
7.0Some Basic Terms
- Hermaphroditism: Single individual produces both eggs and sperms.
- Testosterone: Male sex hormone
- Vas deferens: Sperm duct
- Fallopian tube: Oviduct
- Zygote: Fertilised ovum
- Fertilization: Fusion of male and female gamete
- Placenta: Connecting organ between foetus and uterine wall
- IVF: In Vitro Fertilization
- Oviparous: Animals which lay eggs.
- Viviparous: Animals which give birth to young ones.
8.0Also Read
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