Embryo Development in Mammals

After fertilisation in the oviduct, the zygote divides repeatedly by mitosis as it is moved towards the uterus by cilia and muscle contractions. These early divisions are called cleavage. The cells do not grow between divisions, so the embryo stays about the same size while its cells get smaller with each division: 1 cell → 2 → 4 → 8 → 16.By about day 3 to 4, the embryo is a solid ball of about 16 cells called a morula. It enters the uterus.By about day 5, fluid collects inside the morula and it becomes a hollow ball called a blastocyst. A blastocyst has:
• an outer layer of cells, the trophoblast, which will help to form the placenta.
• a small cluster of cells on one side, the inner cell mass, which will develop into the embryo itself.
• a fluid-filled cavity.About 7 days after fertilisation, the blastocyst embeds in the endometrium (the lining of the uterus). This is implantation. Trophoblast cells grow into the endometrium and absorb nutrients from it, and later, together with maternal tissue, form the placenta that supplies the embryo with oxygen and nutrients and removes waste.The cells of the inner cell mass then begin to specialise, forming the tissues and organs of the fetus.