ICSE Class 8 Biology Chapter 05 Reproduction in Animals

Reproduction In Animals

Syllabus

1. Asexual and sexual reproduction in animals.

- Life cycle of a butterfly - may be observed directly if possible (D).

2. Reproduction in human beings.

3. Physical changes in human beings as a result of growth.

4. Adolescence and adulthood - problems related with adolescence.

- Measuring height and weight of children in different classes - finding averages - tabulating results and arriving at conclusions (group work) (E).

- Observing changes in self - through comparing photographs taken at different ages (E).

- Films on human development, puberty (D).

Introduction to Reproduction

Reproduction is the process of producing new individuals of the same kind. Reproduction also helps in the continuance of the species. Different organisms reproduce in different ways.

You have already studied that some single-celled organisms, like amoeba, reproduce by simply dividing into two. Some multicellular animals like hydra, produce small buds which grow in size and get separated. These are the asexual methods of reproduction (not involving eggs or sperms).

However, most animals reproduce sexually. In this process of reproduction, two sexes produce special types of reproductive cells i.e. males - the sperms and females - the eggs. The sperms and the eggs fuse to form a zygote, which ultimately gives rise to an adult organism.

Asexual Reproduction In Animals

1. Fission (Binary fission or multiple fission) - This is the most common method of reproduction in single-celled organism, e.g., amoeba, etc. It results in the division of the parent amoeba into two daughter amoebae (Fig. 5.1). First, the nucleus divides into two, followed by the division of the cytoplasm and then, the two separate and live as new individuals.

2. Budding - Budding is a process in which buds grow on the outside of the parent body (Fig. 5.2). These buds detach themselves when they are big enough, for independent existence.

3. Regeneration - Animals like hydra and starfish have a great power to regrow the missing or lost parts of their body. This process of generating lost parts is called regeneration. Regeneration can also take place for reproductive purposes. For example, in hydra, if we cut it transversely into 2-3 pieces, each piece will regenerate and give rise to a new individual.

Sexual Reproduction In Animals

Most animals reproduce sexually. In sexual reproduction, the two sexes i.e., male and female produce special types of reproductive cells - sperms in the male and eggs in the female.

To produce a new individual, the sperm has to reach the ovum of the opposite sex and fuse with it. This process is called fertilisation.

After the fertilisation of an ovum by a sperm, a cell called the zygote is formed. The zygote grows by cell division and produces an embryo. The embryo finally grows to become a new individual.

Life Cycle Of An Insect

Different insects have different methods to grow from its egg stage to an adult stage. For example, the egg of a grasshopper hatch out in a tiny hopper called nymph, which feeds and grows into an adult.

But in case of a butterfly, the egg hatches out in a larva, which undergoes transformation to form a pupa. This pupa ultimately gives rise to an Adult. In the case of a butterfly, the transformation of egg to an adult undergoes the following stages:

Egg - Larva - Pupa - Adult

The complete transformation of an egg to an adult butterfly described above is called complete metamorphosis.

Life Cycle of a Butterfly

The butterfly lays the fertilized egg on the leaves. The new born that hatches from the egg is called a larva. It passes through various stages of development. The larva of the butterfly, also called a caterpillar, is active at this stage. It feeds, moves about and grows in size. Once the larva is fully developed, it stops moving and feeding, and develops into pupa which is a resting stage. The larva spins threads around itself to construct a cocoon which protects the underlying pupa. During this period, the pupa develops the features of an adult, though it appears to be at rest. The pupa undergoes reorganisation of tissues and organs and emerges out as a butterfly after breaking open the cocoon.

Activity 3

The life-cycle of a butterfly.

Visit a garden where citrus plants (such as lemon, orange, etc.) are growing. These plants can easily be identified by the peculiar smell of their leaves.

You may find some of these leaves cut up in a semi-circular manner. This is a clear symptom of the activity of a butterfly.

Carefully, look at the plants. You may find tiny yellowish tender eggs sticking on the young tender leaves.

Pluck these leaves, put them in a plastic bag, and bring them to your classroom. Remove some of the eggs from the leaf.

Take a clean wide-mouthed glass bottle or a plastic container. Put some moist sand in it and cover the sand with a piece of blotting paper. Put a few leaves (with eggs) in the container and cover it with a piece of fine cloth. Leave this container at a safe place.

After a few days, the eggs will hatch and the tiny brown coloured caterpillars (larvae) will come out. The caterpillars start feeding on the tender leaves. In the next 2-3 days, the caterpillars will develop irregular white marks on their body surface.

Replace the old and dried leaves with fresh and tender leaves. Pour some water on the blotting paper everyday to keep it moist. Pick up 1 or 2 young caterpillars.

The caterpillars continue to feed, grow in size and moult (shed off their old skin) at regular intervals. Every time the caterpillar moults, it increases in its size. Such moulting occurs 4-5 times.

In about 15-20 days, the caterpillars become fully grown and acquire green colour. These fully grown caterpillars do lot of feeding.

When they stop feeding, they fix themselves on the leaves with a thread secreted by themselves and change into pupa (chrysalis). The larva keep spinning the thread around its body to construct a cocoon. This cocoon protect the pupa whch is a resting stage and does not feed.

After about 10-12 days from this time, the skin of the pupa ruptures, and the adult butterfly comes out, leaving the pupal case hanging empty on the leaf.

The wings of the freshly emerged butterfly are wrinkled and soft. In an hour or so, they become fully stretched and the butterfly begins to fly.

Teacher's Note

Observing butterfly metamorphosis connects classroom learning to nature's cycles, helping students understand how living organisms adapt and transform throughout their lifespans. This direct observation builds curiosity about biological processes they see in their own gardens.

Human Reproductive System

Female - The eggs are produced from the ovaries. The ovaries are two whitish oval bodies, lying within the lower half of the abdomen, one on each side of the uterus. There is an expanded, funnel-shaped tube called oviduct or Fallopian tube close to each ovary. It is through this tube that the egg passes down when released from the ovary.

The two oviducts open into the uterus or womb. The uterus leads to the outside through a muscular tube, called the vagina (popularly known as birth canal). The urine-carrying urethra from the urinary bladder, opens into the vulva just in front of the vaginal opening.

Male - Sperms are produced in the testes. The testes lie outside the abdominal cavity in a sac called the scrotum. In such an arrangment, the testes are at a temperature, 2 - 3°C lower than that of the body. This is the most suitable temperature for sperm production.

The testes consist of a mass of sperm-producing tubes. These tubes join to form ducts leading to the epididymis which in turn leads into a muscular sperm duct. The two sperm ducts, one from each testis, open at the top of the urethra.

The seminal vesicles are a pair of lobulated glands, each opening into the corresponding sperm duct just before it unites with the urethra. The seminal vesicles produce a secretion which serves as a medium for the transportation of the sperms. The mixture of this fluid and the sperms produce a milky fluid, called the semen.

The base of the urethra is surrounded by the prostate gland which pours an alkaline secretion into the semen as it passes through the urethra.

Two small ovoid glands open into the urethra just before it enters the penis. These glands are called Cowper's glands. The secretion of Cowper's glands serves as a lubricant.

The urethra passes through the penis and carries either urine or semen.

Semen is a mixture of sperms and secretions from seminal, prostate and Cowper's glands. It is a milky fluid. Its average amount is 2 - 3 ml in a single ejaculation, and contains 20,000,000 - 40,000,000 sperms.

Fertilisation

During sexual intercourse, the semen gets deposited in the female's vagina. The sperms (present in the semen) released in the vagina actively swim upwards with the help of their tail. Out of the millions of sperms released into the vagina, only a few of them are able to climb up to the upper parts of oviducts. The rest die on the way and are absorbed. If there happens to be an egg in the oviduct, it gets fertilised by just one single sperm. During this fusion, the tail of the sperm is left behind and only the head part (which contains nucleus) penetrates the egg. The nucleus of the sperm fuses with the nucleus of the egg. This is called as fertilisation.

Implantation

The fertilised egg (zygote) soon starts developing and by the time it reaches the uterus, a small ball of numerous cells is already formed. This is a kind of embryo which forms a pit in the wall of the uterus and gets fixed in it. This natural way of fixing of the embryo in the wall of the uterus is called implantation. This produces the state of pregnancy.

By the end of five weeks of pregnancy, the embryo is in quite advanced stage. During this period, the heart and circulatory system are formed. After two months, limbs are also formed.

Growth

Growth means any type of irreversible increase in the size and weight of an individual during the process of development.

In the growth process, a single-celled fertilised egg i.e., a zygote divides and redivides to form a cluster of cells. This division is accompanied by another process i.e., specialisation of cells. The specialisation of cells lead to the formation of different parts of the body. This whole process is called differentiation. This process also gives rise to various tissues, and further, various organs and organ systems. Thus, the zygote grows into an embryo, an embryo into a baby, and a baby ultimately into an adult.

Birth

The full term of the development of an embryo in the uterus is called gestation. In humans, it is about 280 days. At the time of birth, the baby is pushed out with the time of birth, the baby is pushed out with the help oozing out first by the powerful contractions of the muscles of the uterus through vagina.

Teacher's Note

Understanding human reproduction and fetal development helps students appreciate the biological complexity of life and the responsibility that comes with human growth, connecting abstract concepts to their own experience of growing up.

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