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Reproduction in animals




Table of Content

1. Introduction

  • Reproduction is the biological process by which new individual organisms are produced.
  • There are two basic types of reproductions namely asexual and sexual.
  • Asexual reproduction is a process in which the organism reproduces on its own without the help of another individual. There is only single parent.
  • Sexual reproduction is a process in which two parents, a male and a female, are needed to produce a new individual.

2. Reproduction in animals:

  • In most animals, two individuals- a male and a female – are needed for reproduction, therefore the reproduction is sexual.
  • Each individual or parent produces a special sex cell known as gamete. In animals, the gamete produced by the female is called the egg or ovum, and that produced by the male is called the sperm.
  • When the sperm and ovum fuse together, they form the zygote. This is the first cell of the new organism. This process of fusion of a male gamete and a female gamete to form a zygote is called fertilization.
  • The zygote undergoes cellular division and develops into an embryo. The embryo eventually develops into a new individual.
  • In animals such as frogs, fishes, birds, reptiles and humans, an individual carries only one kind of gamete- either sperm (in the male) or the ovum (in the females). However, some organisms produce both male and female gametes in the same body. Such organisms are called hermaphrodites. Snail, slug, leech and earthworm are examples of hermaphrodites.

Types of fertilization:

There are various methods by which fertilization takes place in the living world:

External fertilization:

  • External fertilization is the process of fertilization in which fusion of the gametes takes place outside the body of the organisms.
  • In most aquatic animals, such as bony fish, the male and female release their sperms and eggs into the surrounding water. The sperms swim to the eggs and fertilize them. This fertilization takes place outside the parent’s body.

Internal fertilization:

  • Internal fertilization is the process of fertilization in which fusion of the gametes takes place inside the body of the organisms.
  • In organisms like reptiles, birds and mammals that usually live on land, sperms are released directly into the body of the female since the sperms would be unable to move on land. The males and females mate, and the sperms enter the female body where they fertilize the eggs.

Reproduction in humans:

Fertilization in human is internal.,
Puberty:
The onset of sexual maturity occurs in boys at the age of 11-16 years, and in girls at the age of 10-15 years. This is known as puberty.

The Male reproductive system:

  • The male gametes or sperm are produced in a pair of testes. They are situated outside the abdominal cavity in the scrotal sac.
  • Each sperm is a single cell with the entire usual cell component. Each sperm has a head, a middle piece and a tail.
  • The sperm is carried by a pair of sperm ducts, called vas deferens, to the urethra present in penis.
  • Penis is the organ that transfers the sperms into the female reproductive tract.
  • Sex glands pour a mixture of fluids into the sperm duct that keeps the sperms alive. The sperm and the fluid together form the semen.

Female reproductive system:

  • The female reproductive organs are a pair of ovaries, the oviduct (fallopian tubes) and the uterus.
  • These are located in the lower part of the abdomen.
  • The ovaries produce the egg or ovum when a female reaches puberty.

Fertilization:

  • Male gametes are introduced into the female reproductive tract by the penis. Both the gametes meet in the oviduct. The fusion between a male gamete and a female gamete in the oviduct result in the formation of a fertilized egg or zygote.
  • Fertilization and development of the zygote to form a new individual takes place inside the body of the female.

Development of the fetus:

  • After fertilization, the zygote passes into the uterus, or womb. It divides again and again to form the embryo. The embryo becomes embedded in the lining of the wall of the uterus. This is called implantation.
  • At two months, the embryo starts resembling the human form is called the foetus. It takes nine months for the embryo to grow and develop into a fully-formed baby. This period is called gestation period.
  • The foetus is attached to the uterus by a special structure called the placenta. The placenta is responsible for providing oxygen and nutrients and removing waste substances.

In Vitro fertilization (IVF):

  • It is a method in which ovum collected from a female’s body is allowed to fuse with sperm collected from a male’s body in an external medium or outside the body of the female.
  • The zygote so developed is allowed to grow in vitro (i.e.in glass) for about a week and then implanted in the female’s uterus where it further develops as a normal embryo.
  • A baby born of this technique is often called a ‘test tube’ baby.

Viviparous and oviparous animals:

  • Animals that give birth to young ones are known as viviparous animals.
  • In these animals, the development of the embryo takes place inside the body of the mother, human, tiger, goats and cats are some examples of viviparous animals.
  • Animals that lay eggs are known as oviparous animals. In these animals, the development of embryo takes place in the egg after it is laid. Birds, lizards, crocodiles are some examples of oviparous animals.

Metamorphosis:

    A frog passes through three stages during its development from an egg to an adult frog:
    Development stages of frog
  • Butterflies and moths pass through four stages before winged adult is born:
    metamorphis in butterfly
  • The transformation of larva into an adult through drastic changes is called metamorphosis.

Asexual reproduction:

Asexual reproduction involves the production of new organisms by just one parent. Some of the asexual reproductions are:
Binary fission:
The process by which an organism reproduces by dividing into two cells from a single cell is called binary fission. Binary fission is seen in single-celled organisms, such as Amoeba and Paramecium.
Budding:
In budding, a new organism grows in the form of a bulb-like projection called a bud from the parent organism. It grows and may eventually break away from the parent. Yeast and Hydra reproduce by budding.

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