notes prepared by subhankar Karmakar
Reproduction in Animals:
REPRODUCTION:
The production of new organisms from the existing organisms of the same species is known as reproduction. It is essential for the survival of species on this earth. The process of reproduction ensures continuity of life on earth.
METHODS OF REPRODUCTION:
There are two main methods of reproduction in living organisms.
a. Asexual reproduction
b. Sexual reproduction
Special reproductive cells are called sex cells or gametes.
ASEXUAL REPRODUCTION:
The production of a new organism from a single parent without the involvement of sex cells or gametes is called asexual reproduction.
In asexual reproduction, a part of the parent organism separates of and grows into a new organism. Thus, in asexual reproduction, only one parent is needed to produce a new organism.
Some of the examples of asexual reproduction are binary fission in amoeba and budding in hydra.
SEXUAL REPRODUCTION:
The production of a new organism from two parents by making use of their sex cells or gemetes is called sexual reproduction.
In sexual reproduction, the sex cell of one parent fuses with the sex cell of the other parent to form a new cell called zygote.
This zygote then grows and develops to form a new organism.
Thus, in sexual reproduction, two parents are needed to produce a new organism. The two parents which are involved in sexual reproduction are called male and female.
Male and female parents have special organs in which produce male sex cells and female sex cells respectively.
The humans, fish, frog, cats and dogs all produced by the method of sexual reproduction.
In asexual reproduction, the young one produced is an exact copy of the parent whereas in sexual reproduction, the young one produced is not an exact copy of the parents.
MALE AND FEMALE:
An animal having male sex cells called sperms in its body is called male and having female sex cells called eggs or ova in its body is called female.
GAMETES:
The cells involved in sexual reproduction are called gametes. There are two types of gametes, (a) male gametes and (b) female gametes. The male gametes in animals is called sperm and female gamete in animals is called egg or ovum.
SPERMS:
The sperms are extremely small cells and can be seen only through microscope. It is is about 0.05 millimetre long. It has a head, middle piece and a tail. Sperm is a single cell with all the cell components like nucleus, cytoplasm and cell membrane.
EGGS or OVA:
The eggs or ova are also very small and we need a microscope to see the eggs are ova. The egg or ovum is also a single cell having a nucleus, cytoplasm and a cell membrane. Outside the cell membrane, an egg or ovum has a thin layer of jelly called jelly coat which allows only one sperm to enter into it during fertilisation.
The nuclei of sperm and egg contain chromosomes which carry genes and transmit the characteristics of parents to the offspring. The fusion of a male gamete called sperm with the female gamete called egg gives rise to a new cell called zygote. Thus, the new cell which is formed by the fusion of a male gamete and female gamete is called zygote.
Zygote is a single cell which contains one nucleus. The nucleus of zygote is formed by the combination of nuclei of sperm and egg.
The process of fusion of gametes is called fertilisation.
FERTILISATION:
Sexual reproduction will occur when a male gamete combines with a female gamete. the fusion of male gamete called sperm with female gamete called ovum to form a zygote during sexual reproduction is called fertilisation. Therefore we can say the zygote is actually a fertilised egg or ovum.
All the multicellular animals start their life from a single cell called zygote through sexual reproduction.
There are two types of fertilisation process known as internal fertilisation and external fertilisation.
INTERNAL FERTILISATION:
The fertilisation takes place inside the female body is called internal fertilisation. The process of fertilization in humans the fusion of male gamete of sperm and female gamete or a occurs inside the body of a female animal is called internal fertilisation.
EXTERNAL FERTILISATION:
The fertilization which takes place outside the female body is called external fertilisation. In external fertilisation, the male and female animals released their sperms and eggs in water where fertilisation takes place by collisions between sperms and eggs. For example, the males and females of frogs and fishes release their sperms and eggs in water in which they live. The sperms then collide with the eggs and fertilised them outside the body of female frog or fish.
External fertilisation is very common in aquatic animals such as frog fish and starfish etc.
Although the animals like frog and fish which undergo external fertilisation, lay hundreds of eggs, all the eggs do not get fertilised and hence do not develop into new frogs or fishes.
This is because of the following reasons.
(i) frog of fish lay eggs or release sperms in water. Many of these eggs and sperms are carried away by the movement of water, wind or rainfall and hence all the eggs do not get fertilised.
(ii) many of the eggs of frogs are eaten by other animals which live in water.
(iii) many of the larvae of frog and fish which are hatched from the fertilized eggs are eaten up by other animals in water and fail to develop into adult frogs or fishes.
SEXUAL REPRODUCTION IN ANIMALS
The whole process of sexual reproduction in animals involves the formation of sperms and eggs, joining together of sperm and egg to form a fertilized egg called zygote and growth and development of zygote to form a baby animal.
The sexual reproduction in animals takes place in the following steps:
(a) the male parent produces male gametes called sperms.
(b) the female parent produces female gametes called eggs or ova.
(c) The sperm enters into egg. The nucleus of sperm fuses with the nucleus of egg cell to form a new cell called zygote.
(d) The zygote divides repeatedly to form a hollow ball of hundreds of cells which is called embryo.
(e) Embryo grows and becomes a foetus (in which all the main body features of the baby animal have formed).
(f) Foetus grows and develop to form a new baby animal.
SEXUAL REPRODUCTION IN HUMANS
In human beings, there are special reproductive organs to make sperms and eggs; to bring together sperms and eggs for fertilisation and make a zygote; and for the growth and development of a zygote into a baby.
THE MALE REPRODUCTIVE SYSTEM
The human male reproductive system consists of the following organs: Testes, Scrotal sacs, Epididymis, Sperm ducts, Seminal vesicles and Penis.
Testes are the real reproductive organs in men. Testes make male gametes called sperms. Millions of sperms are produced in the testes. The testes are enclosed in two small bags of skin called scrotal sacs.
The sperms formed in testes come out and go into a coiled tube called Epididymis. From epididymis, the sperms are carried by a long tube called sperm duct into organs called seminal vesicles. The seminal vesicles join to another tube called urethra coming from the bladder. Urethra carries the sperms to an organ called penis which opens outside the body. These sperms are carried in a liquid called semen. When the penis introduces sperms into the woman, one of the sperms be able to fuse with an egg cell and bring about fertilisation.
THE FEMALE REPRODUCTIVE SYSTEM
The human female reproductive system consists of the following organs: Ovaries, Oviducts, Uterus and Vagina.
Ovaries are the real reproductive organs in a woman. Ovaries make the female gametes called eggs or ova. Just above the ovaries, there are tubes called oviducts or fallopian tubes. In human beings, one mature egg is released into oviduct every month by one of the ovaries. The fertilisation of egg by a sperm takes place in the oviduct.
The process of reproduction in humans takes place in two steps:
1. Fertilisation, and
2. Development of embryo.
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