CBSE Class 8 Science, Reproduction in Animals

•   The process of reproduction is not essential for the survival of an individual but reproduction is essential for the continuation of a species.

•   Modes of Reproduction: Like plants, animal also reproduce by (i) Sexual reproduction and (ii) Asexual reproduction.

   Sexual Reproduction: The type of reproduction beginning from the fusion of male and female gametes is known as sexual reproduction. In this process of sexual reproduction, a male and a female gamete (reproductive cells) fuse to form a single cell called Zygote. This zygote gradually develops into an adult, similar to the parents. The individual that grows from a zygote, receives character of both the parents � mother and father.

   Asexual Reproduction: The type of reproduction in which only a single parent is involved is called asexual reproduction. In asexual reproduction, sex cells (gametes) are n6t produced. No fusion of gametes takes place for the production of zygote or off springs.

•   The reproductive parts in humans and the process of reproduction in them. In humans, male and female reproductive parts are present in separate individuals.

Fig. 9.1 Human-male. reproductive organs.

Male Reproductive Organs: Male human reproductive organs contain:

      •   A pair of testes (singular-testis), two sperm duct and a penis.

      •   The testes produce the male gametes� called sperms.

      •   The sperm ducts carry sperms to the penis.

      •   The penis is used for ejecting sperms and also passing urine. The tail in sperm helps in movement of the sperm to reach the egg in the female sex organs.

      •   Human Sperm: A sperm is unicellular (single celled), has a head, a middle piece and a tail.

Fig. 9.2 A human sperm.

Female Reproductive Organs: Female human reproductive organs contain:

      •   A pair of ovaries which produce , female gametes called ova (eggs).

      •   Oviduct (fallopian tubes): A single natured egg is released into the oviduct by one of the ovaries every month.

      •   Uterus: Uterus is the part where development of the baby takes place.

      •   Egg (ovum): Like a sperm, an egg is also a single cell. It contains a nucleus and the cytoplasm.
The egg may be very small as in humans, much larger as in ducks and hens. Ostrich egg is the largest.

Fig. 9.3 Fertilisation.

•   Fertilisation: The fusion of a sperm with an egg is called fertilisation. During fertilisation, the nuclei of the sperm and the egg fuse to form a single nucleus. This results in the formation of a fertilised egg called Zygote.

      Fertilisation may be:

      Internal Fertilisation: Fertilisation which takes place inside the body of the female is called internal fertilisation. For example: as in humans, dogs, cows, birds etc.

      External Fertilisation: Fertilisation which takes place outside the body of the female is called external fertilisation. It is very common in aquatic animals, such as fish, starfish and amphibians (animals who live in water as well as on land such as frog).

      IVF or in vitro Fertilisation: The fertilisation of an egg that takes place outside the body in a test tine or in any other, apparatus is known IVF or in vitro fertilisation.

      Test Tube Babies: In this technique freshly released egg and sperms are put together for a few hours for IMF. In case fertilisation occurs, the zygote is allowed to develop for about a week and then placed in the uterus of the mother. Complete development takes place in the uterus and the baby is born like any other normal baby. The term test tube babies is misleading, because babies cannot grow in test tubes.

•   Embryo: The stage, when cells produced hp the division of the zygote begin to form groups that develop into different tissues and organs of the .body is termed as an embryo.

•   Foetus: The stage of embryo in which al the body parts can be identified is known as foetus.

•   Viviparous: The animals such as dog, lion, elephant, cat etc., which give birth to young ones are called viviparous.

•   Oviparous: The animals, such as lizards, butterfly, crow and hen, which lay eggs that hatch and give rise. to young ones are called oviparous.

•   Life-cycle of Animals: Sexually reproducing animals start their life from a zygote which develops into an embryo that grows into mature adult. The adult produces sex cells (egg and sperm). The fusion of sperm and egg leads to zygote. This whole cyclic order is known as life-cycle.

•   In some animals, the young ones may look very different from the adult. For example, in case of frog, fertilised egg (zygote) develops into tadpole (larva) which later develops into adult frog.

      The tadpole (larva), which is very different from the adult frog and is unable of jumping, transforms into adult frog that is capable of jumping and swimming.

      The process of transformation of the larva into an adult through drastic changes is called metamorphosis.

      Like frog, metamorphosis is also found in sill moth and butterfly.

•   Asexual Reproductjon: The type of reproduction in which only a single parent is involved is called asexual reproduction. For example, budding and binary fission.

      Budding. In ,this case of asexual reproduction, the organism, like hydra, develops a bulge called bud. This bud develops into an adult hydra like structure which when gets separated from the parent hydra leads independent life.

      Another, example of asexual reproduction by budding is seen in yeast.

Fig. 9.6 Budding in Hydra.

      Binary Fission: The process of reproduction in which an animal reproduces by dividing into two individuals known as binary fission. For example, Amoeba asexually reproduces by binary fission.

Fig. 9.7 Binary fissjon in Amoeba

      Cloning: Cloning is the produotion of an exact copy of a cell, any other living part or a complete organism. For example, production of Dolly Sheep by Ian Wilmut and his colleagues in Edinburg, Scotland. Cloning is an artificial method invented by man to produce organisms without sexual or asexual reproduction.