Within few hours after fertilization (while zygote is still in the fallopian tube) the zygote begins a series of mitotic divisions know as cleavage and forms 2, 4, 8, 16 daughter cells called blastomeres. The embryo with 8 to 16 blastomeres is called a morula, a solid ball of cells whose size is the same as that of the zygote. The cells of the morula continue to divide and transform into blastocyst as it moves further into the uterus.
An embryo at the 36 cells stage is called a gastrula. Gastrulation is the process in which it folds in itself to generate two layers and eventually form three layers. The embryo differentiates into an outer layer called ectoderm and an inner layer called endoderm. A mesoderm is soon formed between the ectoderm and the endoderm.
Fertilization takes place in the ampulla of the Fallopian tube through which the embryo travels and implants in the uterus.
Sperms have the ability to reach the site of fertilization within 30 minutes. They have a swimming rate of 1.5 to 3 mm per minute to make this journey.
Majority of the sperms survive only for 48 hrs in the female reproductive system and ovum only for 24 hrs which generates a small time window for the pregnancy to occur.
