Reproduction is the process of producing individuals of the same kind. Most of the organisms reproduce by mating that increases the genetic variability of the organism. The males and females have separate reproductive organs known as gonads. These gonads produce gametes that fuse together to form a single cell called zygote.
Few animals such as earthworms, snails, slugs, etc. are hermaphrodites and possess male and female reproductive organs in the same organism.
Reproduction in animals is of two types:
Sexual Reproduction in Animals
The process in which the male and female gametes fuse together to form a new individual is called sexual reproduction. Let us have a brief account of the human reproductive organs and their role in reproduction.
The male reproductive organs comprise of a pair of testes, sperm ducts, and a penis. The sperms are produced by the testes. The sperms are very small in size with a head, a middle piece, and a tail.
The female reproductive organs comprise of a pair of ovaries, oviducts, and the uterus. The eggs (ova) are produced by the ovaries. The development of the baby takes place in the uterus. A mature egg is released into the oviduct every month.
The semen contains millions of sperms. A single sperm fuses with the ova during fertilization. The nuclei of the egg and the sperm fuse together to form a single nucleus. Thus, a zygote is formed. Fertilization is of two types:
The fertilization that takes place inside the body of the female is known as internal fertilization. For eg., humans, cows, dogs, etc. This method is more prevalent in terrestrial animals. However, some aquatic animals also adopt this method. This may take place by direct introduction of sperms by the male in the female reproductive tract, or the male deposits the sperms in the environment which is picked up by the female and deposited in her reproductive tract.
They are three ways by which offsprings are produced by internal fertilization:
Oviparity– The fertilized eggs are laid outside the body of the female where they receive nourishment from the yolk.
Ovoviviparity– The fertilized eggs are retained in the female’s body where they receive nourishment from the yolk. The eggs are laid right before they are hatched.
Viviparity– The offsprings are born directly instead of hatching from the eggs. They receive nutrition from the mother. This can be seen in mammals.
The fertilization that takes place outside the body of the female is called external fertilization. For eg., frogs, fish. Most fertilization takes place during the process of spawning. The environmental signals such as water temperature trigger spawning.
The zygote divides repeatedly to form a ball of cells. This ball of cells is known as the embryo. These cells differentiate into respective tissues and organs. The embryo gets implanted in the wall of the uterus. This process is known as implantation.
When all the body parts of the embryo start being visible, it is called a foetus. The child is developed after nine months.
Viviparous and Oviparous Animals
|Animals that lay eggs.||Animals who develop inside the mother’s womb.|
|Fertilization can be internal or external.||Fertilization is only internal.|
|Embryo receives nutrients from the egg yolk.||Embryo receives nutrition from the mother.|
|There is very little or no development of the embryo inside the mother.||The embryo develops entirely inside the mother.|
|There are fewer chances of survival since the eggs are laid in an open environment.||The young one is protected inside the mother and so the chances of survival are more.|
|Eg., Insects, hens, fish, amphibians.||Eg., Humans, dogs, cats, horses|
Asexual Reproduction in Animals
Besides sexual reproduction, the other major type of reproduction seen in the animal kingdom is asexual reproduction. This type of reproduction is mostly observed in lower organisms such as single-celled microbes.
What is Asexual Reproduction?
It is the process in which a new individual is formed by the involvement of a single parent with or without the involvement of the gamete formation. The individuals produced are genetically and morphologically similar. It occurs in unicellular organisms. The cells divide by mitotic division and no fertilization takes place. The division occurs very rapidly.
Types of Asexual Reproduction
Asexual Reproduction is of the following types:
It is seen in amoeba and euglena. The parent cell undergoes mitosis and increases in size. The nucleus also divides. Two identical daughter cells are obtained, each containing a nucleus. Prokaryotes like bacteria majorly reproduce by binary fission.
In this, the offspring grows out of the body of the parent. It remains attached to the parent until it matures. After maturation, it detached itself from the parent and lives as an individual organism. This form of reproduction is most common in Hydras.
In some organisms like Planarians, when the body of an organism breaks into several pieces each piece grows into an individual offspring. This is known as fragmentation. It can occur through accidental damage by predators or otherwise, or as a natural form of reproduction. In few animals such as sea star, a broken arm grows into a complete organism.
It is a modified form of fragmentation and occurs mostly in Echinoderms. When a part of an organism, like an arm, detaches from the parent body, it grows into a completely new individual. This is known as regeneration.
This is a form of asexual reproduction where the egg develops without fertilization. This process occurs in bees, wasps, ants, aphids, rotifers, etc. Ants, wasps, and bees produce haploid males. Parthenogenesis has been observed in a few vertebrates such as hammerhead sharks, Komodo dragons, blacktop sharks when the females were isolated from the males.
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