Once fertilization happens, a single-celled zygote starts dividing. The original cell splits into two, then four, then eight, continuing through a process called cleavage. These early divisions happen quickly, where more and more cells get packed into the same space. By the time it reaches about 16 to 32 cells, the embryo forms a compact ball called the morula.

At this stage, each cell is still totipotent, which means it has the potential to become any kind of cell in the body, or even support structures like the placenta. If the embryo splits completely during this stage, the result is identical twins, each forming from the same genetic material.

The next stage is blastulation. Fluid begins to fill the centre of the morula, and it turns into a hollow structure called a blastocyst. This blastocyst has two parts. The inner cell mass will eventually become the embryo itself. The outer layer, called the trophoblast, helps form the placenta, which supports the embryo during pregnancy.

Once the blastocyst reaches the uterus, it attaches to the endometrial lining and begins to implant. Around this time, the cells of the inner mass shift from being totipotent to pluripotent. That means they can now become any type of cell within the embryo, but not support structures like the placenta.

As the embryo grows, it enters a phase called gastrulation. This is where things start getting more organized. Cells begin moving inward through a structure called the primitive streak, and the embryo forms three distinct layers called germ layers:

The ectoderm gives rise to the nervous system and skin. The mesoderm forms muscles, bones, and the circulatory system. The endoderm becomes the inner lining of organs like the lungs and digestive tract.

At this point, cells become multipotent; they can still develop into many cell types, but only within their assigned layer. For example, a cell in the ectoderm might become a neuron or part of the skin, but not a liver cell.

Shortly after gastrulation, the embryo begins neurulation, the first step toward building a nervous system. A flat sheet of cells called the neural plate folds in on itself to form the neural tube, which will later become the brain and spinal cord. This process is guided by the notochord, a rod-like structure that provides support and sends signals to nearby cells to help them differentiate.

Finally, the embryo enters organogenesis, where the basic layers start turning into actual organs. The heart begins forming, limb buds appear, and early versions of the brain, lungs, and kidneys start to take shape.