Final answer:
Three days after fertilization, the fertilized ovum forms a solid mass of cells called a morula through a process called cleavage, and this later develops into a blastocyst as it continues to divide and organize within the uterus.
Step-by-step explanation:
After about 3 days of mitosis, the fertilized ovum forms a solid mass of cells called a morula. This process begins with a single-celled zygote that undergoes multiple rounds of mitosis, with each division called cleavage. The result is the formation of a compact ball of cells that has not increased in overall size since the original zygote, because the cells are confined within the zona pellucida. Once this compacted ball of cells, the morula, reaches the uterus, it continues to divide and transform, eventually forming a structure with a fluid-filled cavity called a blastocyst. Within the blastocyst, an inner cell mass, which will become the embryo, is distinguishable from the outer cells, named trophoblasts, important for forming placental structures for nutrient, waste, and gas exchange.