`star` Fertilization
`star` Determination of sex
`star` Cleavage and Implantation


● During `color{Violet}"copulation"` (coitus) semen is released by the penis into the vagina (`color{Violet}"insemination"`).

● The `color{Violet}"motile sperms"` swim rapidly, pass through the `color{Violet}"cervix"`, enter into the `color{Violet}"uterus"` and finally reach the `color{Violet}"junction"` of the isthmus and ampulla (ampullary-isthmic junction) of the `color{Violet}"fallopian tube"`.

● The ovum released by the ovary is also transported to the `color{Violet}"ampullary-isthmic"` junction where `color{Violet}"fertilisation"` takes place.

● Fertilisation can only occur if the ovum and sperms are transported `color{Violet}"simultaneously"` to the `color{Violet}"ampullary – isthmic junction"`.

● This is the reason why `color{Violet}"not all copulations"` lead to fertilisation and `color{Violet}"pregnancy"`.

● The process of `color{Violet}"fusion"` of a sperm with an ovum is called `color{Violet}"fertilisation"`.

● During fertilisation, a sperm `color{Violet}"comes in contact"` with the `color{Violet}"zona pellucida"` layer of the ovum and induces `color{Violet}"changes"` in the membrane that block the entry of additional sperms (`color{Violet}"polyspermy"`) .

● Thus, it ensures that `color{Violet}"only one sperm"` can fertilise an ovum.

● The secretions of the `color{Violet}"acrosome"` help the sperm enter into the cytoplasm of the ovum through the `color{Violet}"zona pellucida"` and the `color{Violet}"plasma membrane"`.

● This induces the `color{Violet}"completion"` of the meiotic division of the `color{Violet}"secondary oocyte"`.

● The `color{Violet}"second meiotic division"` is also `color{Violet}"unequal"` and results in the formation of a `color{Violet}"second polar body"` and a haploid ovum (`color{Violet}"ootid"`).

● Soon the `color{Violet}"haploid nucleus"` of the sperms and that of the ovum fuse together to form a `color{Violet}"diploid zygote"`.


● As you know the chromosome pattern in the human `color{Violet}"female is XX"` and that in the `color{Violet}"male is XY"`.

●Therefore, all the haploid gametes produced by the `color{Violet}"female (ova)"` have the sex chromosome `color{Violet}"X"` whereas in the `color{Violet}"male gametes (sperms)"` the sex chromosome could be either `color{Violet}"X or Y"`.

● Therefore `color{Violet}"50 per cent"` of sperms carry the `color{Violet}"X chromosome"` while the other 50 per cent carry the`color{Violet}" Y"`.

● After `color{Violet}"fusion"` of the male and female gametes the, zygote either carries `color{Violet}"XX or XY"` depending on whether the `color{Violet}"sperm"` carrying `color{Violet}"X or Y"` fertilized the ovum.

● The zygote carrying `color{Violet}"XX"` develops into a `color{Violet}"female"` baby and `color{Violet}"XY"` would form a `color{Violet}"male"`.

● That is why, scientifically it is correct to say that the `color{Violet}"sex of the baby"` is determined by the `color{Violet}"father"` and not by the mother.


`=>` `color{blue}"Blastomere"` : A blastomere is a type of cell produced by `color{Violet}"cleavage (mitosis)"` of the zygote after fertilization during `color{Violet}"embryonic development"`.

`=>` `color{blue}"Blastocyst"`: The blastocyst is a structure formed in the `color{Violet}"early embryonic development"` of mammals following morula formation. It possesses an `color{Violet}"inner cell mass (ICM)"` which subsequently forms the embryo. The outer layer of the blastocyst consists of cells collectively called the `color{Violet}"trophoblast"`.

`=>` `color{blue}"Morula"` : A morula `color{Violet}"(Latin, morus: mulberry)"` is an early stage embryo consisting of `color{Violet}"8-16 cells (called blastomeres)"` in a solid ball contained within the zona pellucida. it is formed by the cleavage of the zygote and develops further into blastula.

`=>` `color{blue}"Implantation"`: (In reproductive biology of mammals)It means the `color{Violet}"attachment of the fertilized egg"` or blastocyst to the `color{Violet}"wall of the uterus"` at the beginning of pregnancy.

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

● The mitotic division called `color{Violet}"cleavag"`e starts as the `color{Violet}"zygote"` moves through the `color{Violet}"isthmus"` of the oviduct towards the uterus.

● It forms `color{Violet}"2, 4, 8, 16"` daughter cells called `color{Violet}"blastomeres"`.

● The embryo with `color{Violet}"8 to 16 blastomeres"` is called a `color{Violet}"morula"`.

● The `color{Violet}"morula"` continues to divide and transforms into `color{Violet}"blastocyst"` as it moves further into the uterus.

● The `color{Violet}"blastomeres"` in the blastocyst are arranged into an `color{Violet}"outer layer"` called `color{Violet}"trophoblast"` and an inner group of cells attached to trophoblast called the `color{Violet}"inner cell mass"`.

● The `color{Violet}"trophoblast layer"` then gets attached to the `color{Violet}"endometrium"` and the `color{Violet}"inner cell mass"` gets differentiated as the `color{Violet}"embryo"`.

● After attachment, the uterine cells `color{Violet}"divide rapidly"` and cover the `color{Violet}"blastocyst"`. As a result, the blastocyst becomes `color{Violet}"embedded"` `color{Violet}"in the endometrium"` of the uterus.

● This is called `color{Violet}"implantation"` and it leads to `color{Violet}"pregnancy"`.