Theory:

In the last theory, we saw how puberty and sexual reproduction is linked. In this theory, let us see about the different stages of sexual reproduction. Sexual reproduction can be divided into three stages:
 
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Events of sexual reproduction
Pre-fertilisation:
The pre-fertilisation stage involves the events of sexual reproduction that occur before the fusion or fertilisation of male and female gametes.
They include two events:
 
1. Gametogenesis or gamete formation
 
2. Gamete transfer
 
Gametogenesis:
Gametogenesis is the process of the formation of gametes.
Gametes are haploid cells that are morphologically dissimilar (anisogamous) in humans, i.e., the male and female gametes are dissimilar. In humans, the male gamete is called the sperm, and the female gamete is called the egg or ovum.
 
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Sperm and the egg
 
Gamete transfer:
 
The two morphologically distinct gametes must be brought together for fertilisation. In humans, the male gamete is motile, while the female gamete is non-motile. A medium is required through which the male gamete progresses to reach the female gamete.
 
As most male gametes fail to reach the non-motile female gamete, the male gametes are produced in large numbers. They are produced several thousand times than that of the female gamete.
 
Male and female gametes are present in different individuals, and therefore the organism evolves a special mechanism through which gametes are transferred. The transfer of gametes is vital for the process of fertilisation in sexual reproduction.
Fertilisation:
Fertilisation is the fusion of gametes to form a diploid zygote. The process of fertilisation is also referred to as syngamy.
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Fertilisation
  
Where does fertilisation take place?
 
Fertilisation can occur either in the external medium or internally inside the body of an organism. Thus, there are two types of fertilisation, namely:
 
1. External fertilisation
 
2. Internal fertilisation
 
In humans and higher animals, fertilisation occurs internally. The fusion of gametes takes place inside the body of the female. The male gamete is mobile and reaches the egg to fuse with it and form a zygote.
Post-fertilisation:
Post-fertilisation events are that take place in the sexual reproduction following the fertilisation.
The post-fertilisation events include:
 
1. Zygote development
 
2. Embryogenesis
 
Zygote:
 
A zygote is a diploid and the fertilisation product in sexual reproduction. It serves as a link between one generation and the next generation. In the case of external fertilisation, the zygote is formed externally in the medium. In internal fertilisation, the zygote is formed inside the female's body.
 
Embryogenesis:
Embryogenesis is the process of the development of an embryo from the zygote.
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Development of an embryo in humans
 
During the process of embryogenesis, the zygote undergoes mitosis and cell differentiation. Cell division increases the number of cells in the embryo. Cell differentiation helps in the formation of specialised tissues and organs in an organism.
 
Based on zygote's development, animals are classified into:
  • Oviparous (zygote develops outside the organism)
  • Viviparous (zygote develops inside the organism)
Humans are viviparous as zygote development occurs in the female's body.
Reference:
https://commons.wikimedia.org/wiki/File:The_sperm_and_ovum_during_fertilization.svg
https://pixabay.com/illustrations/ovum-sperm-egg-cell-male-female-5818117/
https://www.freepik.com/free-vector/stages-human-embryonic-development_10804975.htm