Updated by: Arabinda Naik
The unicellular zygote undergoes cleavage to form a solid ball of cells called morula. The morula is formed of 16 to 32 cells. It is not larger than the zygote. The morula reaches uterus 4-6 days after fertilization. It is still surrounded by zona pellucida that never allows its attachment to the abnormal side of uterus. Cleavage increases cell number but does not result in growth. It restores the cell size and nucleocytoplasmic ratio, which is a characteristic of a species.