Changes

[[Image:Blastocyst.gif|thumb|right|300px|Blastocyst - Courtesy of Wikimedia Commons]]

[[Image:Blastocyst.gif|thumb|right|300px|Blastocyst - Courtesy of Wikimedia Commons]]

* The sperm enters the the oocyte forming the ootid.

* The sperm enters the the oocyte forming the ootid.

* In the Ootid, male and female '''pronuclei''' along with the first and second '''polar bodies''' are present.

* In the ootid, male and female '''pronuclei''' along with the first and second '''polar bodies''' are present.

* Fusion of the male and female pronuclei results in a single '''diploid nucleus''' ('''Syngamy''')

* Fusion of the male and female pronuclei results in a single '''diploid nucleus''' ('''syngamy''')

* The '''zona pellucida''' then becomes an imprenetrable layer that prevents polyspermy and so polyploidy.  

* The '''zona pellucida''' then becomes an imprenetrable layer that prevents polyspermy and so polyploidy.  

* '''Zygote''' (diploid) undergoes mitotic divisions ('''cleavage''').

* '''Zygote''' (diploid) undergoes mitotic divisions ('''cleavage''').

* Cleavage gives rise to daughter cells called '''Blastomeres'''.

* Cleavage gives rise to daughter cells called '''blastomeres'''.

* Cleavage divisions continue and the 4-celled Embryo gives rise to an 8-celled Embryo.

* Cleavage divisions continue and the 4-celled embryo gives rise to an 8-celled embryo.

* These cells are totipotent and divide asynchronously.

* These cells are totipotent and divide asynchronously.

* After the 8-celled stage, a ball of cells is formed.  This stage is referred to as a '''Morula'''.

* After the 8-celled stage, a ball of cells is formed.  This stage is referred to as a '''morula'''.

* '''Tight junctions''' form between outer cells of the Morula.

* '''Tight junctions''' form between outer cells of the morula.

* '''Gap junctions''' form between inner cells of the Morula.

* '''Gap junctions''' form between inner cells of the morula.

* Thus, two groups of cells are created within the Morula.

* Thus, two groups of cells are created within the morula.

* Outer cells pump Sodium into the intracellular spaces and water follows osmotically.

* Outer cells pump sodium into the intracellular spaces and water follows osmotically.

* Fluid begins to accumulate within the Morula.

* Fluid begins to accumulate within the morula.

* Cells  of the Morula continue to divide and a '''Blastocyst''' develops, consisting of:

* Cells  of the morula continue to divide and a '''blastocyst''' develops, consisting of:

** '''Inner Cell Mass''' (ICM)

** '''Inner cell mass''' (ICM)

** '''Blastocoele''' cavity

** '''Blastocoele''' cavity

** Single layer of '''Trophoblast''' cells

** Single layer of '''trophoblast''' cells

* As fluid accumulates, the outer cells become flattened and a cavity known as the Blastocoele is formed.

* As fluid accumulates, the outer cells become flattened and a cavity known as the blastocoele is formed.

* Gap junctions connecting the inner cells of the Morula enable them to polarise as a group.

* Gap junctions connecting the inner cells of the morula enable them to polarise as a group.

* As a result, two separate cellular components emerge - the Inner Cell Mass and the Trophoblasts.

* As a result, two separate cellular components emerge - the inner cell mass and the trophoblasts.