Changes

== Pre-Implantation Development ==

== Pre-Implantation Development ==

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

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

* The sperm enters the the oocyte forming the ootid.

Once sperm has entered the the '''oocyte''', an '''ootid''' is formed. At an early stage the ootid will contain male and female '''pronuclei''' along with the first and second '''polar bodies'''. Fusion of the male and female '''pronuclei''' will result in a single '''diploid nucleus''' or '''syngamy'''. Once this '''syngamy''' has occurred, the '''zona pellucida''' then develops into  an imprenetrable layer that prevents polyspermy and so polyploidy. Once the '''zona pellucida''' has developed, the ootid is now referred to as a '''zygote''' (diploid) and will begin undergoing mitotic divisions via a '''cleavage''' process that will begin to  give rise to daughter cells called '''blastomeres'''. These cleavage divisions will begin to produce a 4-celled embryo and then an 8-celled embryo.

* 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''')

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

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

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

* 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'''.