`star` Fertilisation
`star` Types of Fertilisation
A) External Fertilisation
B) Internal Fertilisation
`star` Post-Fertilisation Events
A) Zygote
B) Embryogenesis


`color{blue}"Syngamy"`: Syngamy can be defined as the process of `color{violet}"fusion"` of egg nucleus with male gamete to form a `color{violet}"diploid zygote"` during `color{violet}"fertilization"`. The terms syngamy and fertilisation are frequently used though, `color{violet}"interchangeably"`.

`color{blue}"Parthenogenesis"`: Parthenogenesis is a natural form of `color{violet}"asexual reproduction"` in which growth and development of `color{violet}"embryos occur without fertilization"`. In animals, parthenogenesis means development of an embryo from an `color{violet}"unfertilized egg cell"`.

`color{blue}"Fertilisation"`: The process involving the `color{violet}"fusion of male and female"` gametes to form a `color{violet}"zygote"`.

`color{green} ✍️ color{green} mathbf("KEY CONCEPT")`

● The most vital event of sexual reproduction is perhaps the `color{Violet}"fusion of gametes"`.

● This process called `color{Violet}"syngamy"` results in the formation of a`color{Violet}" diploid zygote"`. The term `color{Violet}"fertilisation"` is also often used for this process.

● The terms `color{Violet}"syngamy and fertilisation"` are frequently used though, `color{Violet}"interchangeably"`.

● In some organisms like rotifers, honeybees and even some lizards and birds (turkey), the `color{Violet}"female gamete"` undergoes `color{Violet}"development"` to form new organisms `color{Violet}"without fertilisation"`.

● This phenomenon is called `color{Violet}"parthenogenesis"`.


`color{blue}"External Fertilisation"`: The type of `color{violet}"gametic fusion"` in most aquatic organisms where syngamy occurs in the `color{violet}"external medium"` (water), i.e., `color{violet}"outside the body"` of the organism.

`color{green} ✍️ color{green} mathbf("KEY CONCEPT")`

● In most `color{Violet}"aquatic organisms"`, such as a majority of algae and fishes as well as amphibians, syngamy occurs in the `color{Violet}"external medium (water)"`, i.e., outside the body of the organism.

● This type of `color{Violet}"gametic fusion"` is called `color{Violet}"external fertilisation"`.

● Organisms exhibiting external fertilisation show `color{Violet}"great synchrony"` between the `color{Violet}"sexes"`.

● They release a `color{Violet}"large number of gametes"` into the `color{Violet}"surrounding medium"` (water) in order to `color{Violet}"enhance the chances"` of syngamy.

●This happens in the bony fishes and frogs where a `color{Violet}"large number of offspring"` are produced.

● A major disadvantage is that the `color{Violet}"offspring are extremely vulnerable to predators"` threatening their survival up to adulthood.


`color{blue}"Internal Fertilisation"`: The type of `color{violet}"gametic fusion"` in most terrestrial organisms where syngamy occurs `color{violet}"inside the body"` of the organism.

`color{green} ✍️ color{green} mathbf("KEY CONCEPT")`

● In many `color{Violet}"terrestrial organisms"`, belonging to fungi, higher animals such as reptiles birds, mammals and in a majority of plants (bryophytes, pteridophytes, gymnosperms and angiosperms), syngamy occurs `color{Violet}"inside the body"` of the organism.

● This process is called `color{Violet}"internal fertilisation"`.

● In all these organisms, `color{Violet}"egg"` is formed `color{Violet}"inside the female body"` where they fuse with the `color{Violet}"male gamete"`.

●In organisms exhibiting internal fertilisation, the `color{Violet}"male gamete is motile"` and has to `color{Violet}"reach the egg"` in order to fuse with it.

● In these even though the number of `color{Violet}"sperms"` produced is `color{Violet}"very large"`, there is a significant `color{Violet}"reduction"` in the `color{Violet}"number of eggs"` produced.

● In `color{Violet}"seed plants"`, however, the `color{Violet}"non-motile"` male gametes are carried to female gamete by `color{Violet}"pollen tubes"`.


● Events in sexual reproduction after the formation of zygote are called `color{Violet}"post-fertilisation events"`.

`color{Blue}"A) The Zygote"`
● Formation of the `color{Violet}"diploid zygote"` is `color{Violet}"universal"` in all sexually reproducing organisms.

● In organisms with external fertilisation, zygote is formed in the `color{Violet}"external medium"` (usually water), whereas in those exhibiting internal fertilisation, zygote is formed `color{Violet}"inside the body"` of the organism.

● Further development of the zygote depends on the `color{Violet}"type of life cycle"` the organism has and `color{Violet}"the environment"` it is exposed to.

`star` `color{Brown}"In Fungi and Algae"`
● In organisms belonging to `color{Violet}"fungi and algae"`, zygote develops a `color{Violet}"thick wall"` that is resistant to `color{Violet}"dessication and damage"`.

● It undergoes a period of `color{Violet}"rest before germination"`.

`star` `color{Brown}"In organisms with Haplontic life cycle"`

● In organisms with `color{Violet}"haplontic life cycle"`, zygote divides by meiosis to form `color{Violet}"haploid spores"` that grow into haploid individuals.

● Zygote is the `color{Violet}"vital link"` that ensures `color{Violet}"continuity of species"` between organisms of one generation and the next.

● Every `color{Violet}"sexually reproducing"` organism, including human beings begin life as a `color{Violet}"single cell–the zygote"`.


`color{blue}"Oviparous Animals"`: Those animals which produce young ones by `color{violet}"means of eggs"` which are `color{violet}"hatched"` after they have been `color{violet}"laid by the parent"`, as in birds.

`color{blue}"Viviparous animals"`: Those animals which `color{violet}"deliver live young ones"` which have `color{violet}"developed inside"` the body of the parent.

`color{blue}"Embryogenesis"`: Embryogenesis refers to the process of `color{violet}"development of embryo"` from the `color{violet}"zygote"`

`color{green} ✍️ color{green} mathbf("KEY CONCEPT")`

● `color{Violet}"Embryogenesis"` refers to the process of `color{Violet}"development"` of `color{Violet}"embryo from the zygote. "`

● During embryogenesis, zygote undergoes `color{Violet}"cell division"` (mitosis) and `color{Violet}"cell differentiation"`.

● While `color{Violet}"cell divisions increase"` the number of cells in the developing embryo; `color{Violet}"cell differentiation"` helps groups of cells to undergo `color{Violet}"certain modifications"` to form `color{Violet}"specialised"` tissues and organs to form an organism.

● Animals are categorised into `color{Violet}"oviparous and viviparous"` based on whether the `color{Violet}"development of the zygote"` take place outside the body of the female parent or inside, i.e., whether they lay `color{Violet}"fertilised/unfertilised eggs"` or give birth to `color{Violet}"young ones"`.

`star` `color{Brown}"In Oviparous Animals"`:
● In `color{Violet}"oviparous animals"` like reptiles and birds, the `color{Violet}"fertilised eggs"` covered by `color{Violet}"hard calcareous shell"` are laid in a safe place in the environment.

● After a `color{Violet}"period of incubation"` young ones hatch out.

`star` `color{Brown}"In Viviparous Animals"`:
● In `color{Violet}"viviparous animals"` (majority of mammals including human beings), the zygote develops into a `color{Violet}"young one"` `color{Violet}"inside the body"` of the female organism.

● After attaining a `color{Violet}"certain stage of growth"`, the young ones are `color{Violet}"delivered"` out of the body of the female organism.

● Because of `color{Violet}"proper"` `color{Violet}"embryonic care"` and `color{Violet}"protection"`, the `color{Violet}"chances of survival"` of young ones is greater in viviparous organisms.

`star` `color{Brown}"In Flowering Plants"`
● In `color{Violet}"flowering plants"`, the zygote is formed `color{Violet}"inside the ovule"`. After fertilisation the sepals, petals and stamens of the flower `color{Violet}"wither and fall"` off.

● The `color{Violet}"pistil"` however, `color{Violet}"remains attached"` to the plant.

● The `color{Violet}"zygote"` develops into the `color{Violet}"embryo"` and the `color{Violet}"ovules"` develop into the `color{Violet}"seed"`.

● The `color{Violet}"ovary"` develops into the `color{Violet}"fruit"` which develops a `color{Violet}"thick wall called pericarp"` that is protective in function.

● After dispersal, seeds germinate under favourable conditions to produce new plants.