As the zygote travels down the Fallopian tube, it undergoes cleavage, leading to the formation of a morula and eventually a blastocyst before reaching the uterus for implantation.
As the fertilized egg, or zygote, moves down the Fallopian tube toward the uterus, a crucial process called cleavage is occurring. This is where the zygote undergoes many rapid cell divisions, multiplying from one cell into two, then four, and so on. This stage of embryonic development leads to the formation of a ball of cells known as a morula, and later, into the blastocyst stage as it approaches the uterus.
The process of implantation, where the blastocyst attaches to the uterine wall, only occurs once the blastocyst reaches the inside of the uterus. Therefore, options a, b, and c provided in the question all relate to events that occur after the fertilized egg has completed its journey through the Fallopian tube, whereas option d, division into identical twins, can occur at this early stage but is not a guaranteed event for every zygote.