Reproduction
It is one of the important life processes, which ensures the continuation of similar kinds of individuals (species) generation after generation.
Modes of Reproduction
There are two modes by which animals reproduce:
Sexual Reproduction
The process of reproduction, which results from the fusion of male and female gametes is called sexual reproduction.
Male Reproductive Organs
Male reproductive organs are a pair of testes, sperm ducts, and a penis. The testes produce the male gametes called sperms.
Female Reproductive Organs
The female reproductive organs are a pair of ovaries, oviducts (fallopian tubes) and the uterus.
Ovary produces females gametes called ova (Egg).
Fertilisation
In human beings, a single matured egg is released into the oviduct by one of the ovaries every month.
Uterus is the part of the female reproductive system where the development of the embryo takes place.
Both sperm and ova are single-celled structure.
The fusion of the ovum and the sperm is called fertilisation.
1. Internal Fertilisation
Fertilisation that takes place inside the female body is called internal fertilisation. This is observed in human beings and other animals such as cows and dogs.
2. External Fertilisation
Fertilisation that takes place outside the female body is called external fertilisation. This is common in aquatic animals such as frogs, fish, starfish, etc.
During fertilisation, the nuclei of the sperm (n) and the egg in) are fused to form a single nucleus (2n). This fertilised egg is called zygote.
The zygote divides repeatedly to give rise to a ball (mass) of cells.
The cells then begin to differentiate into various tissues.
This developing structure is called an embryo .
From the above figure:
(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)
The embryo gets implanted within the wall of the uterus, i.e., endometrium for further development. The stage of the embryo in which all the body parts are identifiable is called a foetus.
The animals which give birth to young ones are called viviparous animals.
The animals which lay eggs are called oviparous animals.
The transformation of larva into an adult through drastic changes is called metamorphosis.
Asexual Reproduction
The type of reproduction in which only a single parent is involved, is called asexual reproduction.
Types of Asexual Reproduction
In small animals like a hydra, new individuals develop from buds. This method of asexual reproduction is called budding.
1. Bud
A lateral outgrowth from the parent body that assumes the shape of the parent. It ultimately gets detached and behaves as a new individual.
2. Binary Fission
In binary fission, a single-celled individual reproduces by dividing itself into two. Example: Amoeba.
3. Tadpoles
In the life process of a frog, we find three distinct stages, that is egg →tadpole → adult.
These tadpoles get transformed into adults which are capable of jumping and swimming and are finally transformed into a frog.
4. Cloning
Cloning is the creation of an organism that is an exact genetic copy of another. This means that every single bit of DNA is the same between the two organisms.
Dolly was the first mammal to be cloned.